How to get Cause by value in groovy-html.template

While entering build_user, it is giving error 

Exception raised during template rendering: No such property: user for class: SimpleTemplateScript10 groovy.lang.MissingPropertyException: No such property: user for class: SimpleTemplateScript10 at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:53) at org.codehaus.groovy.runtime.callsite.PogoGetPropertySite.getProperty(PogoGetPropertySite.java:52) at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callGroovyObjectGetProperty(AbstractCallSite.java:307) at SimpleTemplateScript10.run(SimpleTemplateScript10.groovy:1) at groovy.text.SimpleTemplateEngine$SimpleTemplate$1.writeTo(SimpleTemplateEngine.java:168) at groovy.text.SimpleTemplateEngine$SimpleTemplate$1.toString(SimpleTemplateEngine.java:180) at hudson.plugins.emailext.plugins.content.ScriptContent.renderTemplate(ScriptContent.java:151) at hudson.plugins.emailext.plugins.content.ScriptContent.evaluate(ScriptContent.java:81) at hudson.plugins.emailext.plugins.content.AbstractEvalContent.evaluate(AbstractEvalContent.java:76) at org.jenkinsci.plugins.tokenmacro.DataBoundTokenMacro.evaluate(DataBoundTokenMacro.java:202) at org.jenkinsci.plugins.tokenmacro.Parser.processToken(Parser.java:323) at