when I test connection on slack notification configuration, I always get `failure` message. Becise that there is no other messages explain why it failed. In system log I can see below error. 

```

Caught exception evaluating: descriptor.successResponseCodesCsv in /configure. Reason: java.lang.reflect.InvocationTargetException java.lang.NullPointerException at hudson.plugins.sitemonitor.mapper.SuccessCodeListToCvString.apply(SuccessCodeListToCvString.java:41) at hudson.plugins.sitemonitor.SiteMonitorDescriptor.getSuccessResponseCodesCsv(SiteMonitorDescriptor.java:125) Caused: java.lang.reflect.InvocationTargetException at sun.reflect.GeneratedMethodAccessor3148.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.apache.commons.jexl.util.PropertyExecutor.execute(PropertyExecutor.java:125) at org.apache.commons.jexl.util.introspection.UberspectImpl$VelGetterImpl.invoke(UberspectImpl.java:314) at org.apache.commons.jexl.parser.ASTArrayAccess.evaluateExpr(ASTArrayAccess.java:185) at org.apache.commons.jexl.parser.ASTIdentifier.execute(ASTIdentifier.java:75) at org.apache.commons.jexl.parser.ASTReference.execute(ASTReference.java:83) at org.apache.commons.jexl.parser.ASTReference.value(ASTReference.java:57) at org.apache.commons.jexl.parser.ASTReferenceExpression.value(ASTReferenceExpression.java:51) at org.apache.commons.jexl.ExpressionImpl.evaluate(ExpressionImpl.java:80) at hudson.ExpressionFactory2$JexlExpression.evaluate(ExpressionFactory2.java:74) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:58) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.WhenTag.doTag(WhenTag.java:46) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.ChooseTag.doTag(ChooseTag.java:38) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.JellyViewScript.run(JellyViewScript.java:95) at org.kohsuke.stapler.jelly.IncludeTag.doTag(IncludeTag.java:147) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.OtherwiseTag.doTag(OtherwiseTag.java:41) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.ChooseTag.doTag(ChooseTag.java:38) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.TagSupport.invokeBody(TagSupport.java:161) at org.apache.commons.jelly.tags.core.ForEachTag.doTag(ForEachTag.java:150) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$1.run(CoreTagLibrary.java:98) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.CallTagLibScript$1.run(CallTagLibScript.java:99) at org.apache.commons.jelly.tags.define.InvokeBodyTag.doTag(InvokeBodyTag.java:91) at org.apache.commons.jelly.impl.TagScript.run(TagScript.java:269) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.kohsuke.stapler.jelly.ReallyStaticTagLibrary$1.run(ReallyStaticTagLibrary.java:99) at org.apache.commons.jelly.impl.ScriptBlock.run(ScriptBlock.java:95) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.CallTagLibScript.run(CallTagLibScript.java:120) at org.apache.commons.jelly.tags.core.CoreTagLibrary$2.run(CoreTagLibrary.java:105) at org.kohsuke.stapler.jelly.JellyViewScript.run(JellyViewScript.java:95) at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:63) at org.kohsuke.stapler.jelly.DefaultScriptInvoker.invokeScript(DefaultScriptInvoker.java:53) at org.kohsuke.stapler.jelly.JellyFacet$1.dispatch(JellyFacet.java:97) at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:715) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:845) at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649) at org.kohsuke.stapler.Stapler.service(Stapler.java:238) at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1669) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:135) at org.jenkinsci.plugins.ssegateway.Endpoint$SSEListenChannelFilter.doFilter(Endpoint.java:225) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132) at io.jenkins.blueocean.auth.jwt.impl.JwtAuthenticationFilter.doFilter(JwtAuthenticationFilter.java:51) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132) at com.smartcodeltd.jenkinsci.plugin.assetbundler.filters.LessCSS.doFilter(LessCSS.java:47) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132) at io.jenkins.blueocean.ResourceCacheControl.doFilter(ResourceCacheControl.java:134) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132) at hudson.plugins.scm_sync_configuration.extensions.ScmSyncConfigurationFilter$1.call(ScmSyncConfigurationFilter.java:49) at hudson.plugins.scm_sync_configuration.extensions.ScmSyncConfigurationFilter$1.call(ScmSyncConfigurationFilter.java:44) at hudson.plugins.scm_sync_configuration.ScmSyncConfigurationDataProvider.provideRequestDuring(ScmSyncConfigurationDataProvider.java:106) at hudson.plugins.scm_sync_configuration.extensions.ScmSyncConfigurationFilter.doFilter(ScmSyncConfigurationFilter.java:44) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132) at jenkins.metrics.impl.MetricsFilter.doFilter(MetricsFilter.java:125) at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:132) at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:138) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:49) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84) at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at jenkins.security.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:117) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:92) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249) at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:67) at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87) at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:90) at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:171) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30) at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652) at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143) at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:553) at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223) at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127) at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515) at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185) at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061) at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141) at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97) at org.eclipse.jetty.server.Server.handle(Server.java:499) at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311) at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257) at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544) at winstone.BoundedExecutorService$1.run(BoundedExecutorService.java:77) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:748)
Jul 31, 2017 10:45:05 PM WARNING jenkins.plugins.slack.StandardSlackService publish
Slack post may have failed. Response: <html><head><meta http-equiv='refresh' content='1;url=/login?from=%2FOPRc9G4zB2JX289VOnTvfeey'/><script>window.location.replace('/login?from=%2FOPRc9G4zB2JX289VOnTvfeey');</script></head><body style='background-color:white; color:white;'> Authentication required <!-- You are authenticated as: anonymous Groups that you are in: Permission you need to have (but didn't): hudson.model.Hudson.Read ... which is implied by: hudson.security.Permission.GenericRead ... which is implied by: hudson.model.Hudson.Administer --> </body></html>
Jul 31, 2017 10:45:05 PM WARNING jenkins.plugins.slack.StandardSlackService publish
Response Code: 403
Jul 31, 2017 10:45:09 PM WARNING jenkins.plugins.slack.StandardSlackService publish
Slack post may have failed. Response: <html><head><meta http-equiv='refresh' content='1;url=/login?from=%2FOPRc9G4zB2JX289VOnTvfeey'/><script>window.location.replace('/login?from=%2FOPRc9G4zB2JX289VOnTvfeey');</script></head><body style='background-color:white; color:white;'> Authentication required <!-- You are authenticated as: anonymous Groups that you are in: Permission you need to have (but didn't): hudson.model.Hudson.Read ... which is implied by: hudson.security.Permission.GenericRead ... which is implied by: hudson.model.Hudson.Administer --> </body></html>
Jul 31, 2017 10:45:09 PM WARNING jenkins.plugins.slack.StandardSlackService publish
Response Code: 403

```