In the project configuration of a job, when you don't select Mantis Project and/or Category.
and choose to activate the option : "File a ticket to Mantis"
a NullPointerException is thrown

java.lang.NullPointerException
at hudson.plugins.mantis.soap.mantis120.MantisSessionImpl.addIssue(MantisSessionImpl.java:134)
at hudson.plugins.mantis.MantisSite.addIssue(MantisSite.java:173)
at hudson.plugins.mantis.MantisIssueRegister.perform(MantisIssueRegister.java:74)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:703)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:678)
at hudson.ivy.IvyModuleSetBuild$RunnerImpl.post2(IvyModuleSetBuild.java:587)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:625)
at hudson.model.Run.run(Run.java:1435)
at hudson.ivy.IvyModuleSetBuild.run(IvyModuleSetBuild.java:282)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:238)

The problem become from the class : MantisIssueRegister.createIssue
Where this test exist :
if (projectId == MantisProject.NONE || MantisCategory.None.equals(categoryName)) {
return null;
}

Is it possible to have just a log message maybe first.
And after, a required field on project configuration and the ability to have category field : not selected
if there is no impact on the rest of the plugin.