Install Build Monitor Plugin as it will make triggering the error easier: https://wiki.jenkins-ci.org/display/JENKINS/Build+Monitor+Plugin

To replicate the problem first make Jenkins throw a NullPointerException.
On Jenkins 1.480 this can be easily done by triggering JENKINS-15309 (https://issues.jenkins-ci.org/browse/JENKINS-15309)

• create a new Build Monitor view and add some jobs to it. Shut down Jenkins.
• edit the config.xml file and make sure that instead of <jobNames class="tree-set"> you have <jobNames> (no class specified) as this will cause an unhandled NullPointerException
• start up Jenkins and navigate to the Build Monitor view tab
• open Google Chrome developer console, go to Network Tab and view the response returned by Jenkins to a fetchJobViews request. It will be something like the following:

HEADERS:
Request URL:http://localhost:8080/$stapler/bound/c9717585-a435-44f6-84fc-732ac9460461/fetchJobViews
Request Method:POST
Status Code:200 OK
Request Headersview source

BODY:
cause: null
localizedMessage: ""
message: ""
stackTrace: [

{className:java.util.TreeSet, fileName:TreeSet.java, lineNumber:154, methodName:<init>,…}

,…]

Please note the "200 OK" part. If there is an internal server error, Jenkins should never return a 200 OK, but something more appropriate such as 500.
Returning a correct HTTP error code will make it easier for plugin developers to correctly react the the problem.