Stack trace
javax.servlet.ServletException: java.lang.NoSuchFieldError: INSTANCE
	at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:796)
	at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
	at org.kohsuke.stapler.MetaClass$6.doDispatch(MetaClass.java:249)
	at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
	at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
	at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
	at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
	at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
	at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:491)
	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1074)
	at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:123)
	at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:114)
	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1065)
	at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:48)
	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1065)
	at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
	at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
	at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:168)
	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1065)
	at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:49)
	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1065)
	at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1065)
	at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
	at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1065)
	at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:365)
	at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:185)
	at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
	at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:689)
	at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:391)
	at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:146)
	at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
	at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:139)
	at org.mortbay.jetty.Server.handle(Server.java:285)
	at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:457)
	at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:765)
	at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:628)
	at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:203)
	at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:357)
	at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:329)
	at org.mortbay.thread.BoundedThreadPool$PoolThread.run(BoundedThreadPool.java:475)
Caused by: java.lang.NoSuchFieldError: INSTANCE
	at org.apache.http.impl.io.DefaultHttpRequestWriterFactory.<init>(DefaultHttpRequestWriterFactory.java:52)
	at org.apache.http.impl.io.DefaultHttpRequestWriterFactory.<init>(DefaultHttpRequestWriterFactory.java:56)
	at org.apache.http.impl.io.DefaultHttpRequestWriterFactory.<clinit>(DefaultHttpRequestWriterFactory.java:46)
	at org.apache.http.impl.conn.ManagedHttpClientConnectionFactory.<init>(ManagedHttpClientConnectionFactory.java:72)
	at org.apache.http.impl.conn.ManagedHttpClientConnectionFactory.<init>(ManagedHttpClientConnectionFactory.java:84)
	at org.apache.http.impl.conn.ManagedHttpClientConnectionFactory.<clinit>(ManagedHttpClientConnectionFactory.java:59)
	at org.apache.http.impl.conn.PoolingHttpClientConnectionManager$InternalConnectionFactory.<init>(PoolingHttpClientConnectionManager.java:487)
	at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.<init>(PoolingHttpClientConnectionManager.java:147)
	at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.<init>(PoolingHttpClientConnectionManager.java:136)
	at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.<init>(PoolingHttpClientConnectionManager.java:112)
	at org.apache.http.impl.client.HttpClientBuilder.build(HttpClientBuilder.java:726)
	at jenkins.plugins.slack.StandardSlackService.getHttpClient(StandardSlackService.java:200)
	at jenkins.plugins.slack.StandardSlackService.publish(StandardSlackService.java:132)
	at jenkins.plugins.slack.StandardSlackService.publish(StandardSlackService.java:95)
	at jenkins.plugins.slack.SlackNotifier$DescriptorImpl.doTestConnection(SlackNotifier.java:514)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:298)
	at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:161)
	at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:96)
	at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:121)
	at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
	at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
	... 41 more