How to reproduce with clear Hudson installation (with only two related plugins additionally installed):
1. Create free style software project A, in Join Trigger section set to build downstream project B.
2. Create free style software project B.
3. Start A job.
A job will succeed, but in it's log will be shown exception (http://HUDSON/job/A/1/console):

Started by user anonymous
ERROR: Publisher join.JoinTrigger aborted due to exception
java.lang.NullPointerException
at join.JoinTrigger.TryGetParameterizedDownstreamNames(JoinTrigger.java:94)
at join.JoinTrigger.perform(JoinTrigger.java:82)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:583)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:564)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:551)
at hudson.model.Build$RunnerImpl.post2(Build.java:152)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:535)
at hudson.model.Run.run(Run.java:1202)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:122)
Notifying upstream projects of job completion
Finished: SUCCESS