If you need to specify the build output directory, but failed to do so, you will get a very bad exception:

Packaging IPA
FATAL: null
java.lang.NullPointerException
at au.com.rayh.XCodeBuilder.perform(XCodeBuilder.java:440)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:717)
at hudson.model.Build$BuildExecution.build(Build.java:199)
at hudson.model.Build$BuildExecution.doRun(Build.java:160)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:499)
at hudson.model.Run.execute(Run.java:1502)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:236)