Happens to me on Version 0.10.1 of the plugin

ERROR: Publisher hudson.plugins.mantis.MantisIssueUpdater aborted due to exception
java.lang.NullPointerException
at hudson.plugins.mantis.Updater.findChangeSetsFromSCM(Updater.java:136)
at hudson.plugins.mantis.Updater.findChangeSets(Updater.java:120)
at hudson.plugins.mantis.Updater.perform(Updater.java:55)
at hudson.plugins.mantis.MantisIssueUpdater.perform(MantisIssueUpdater.java:53)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:630)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:608)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:584)
at hudson.model.Build$RunnerImpl.post2(Build.java:159)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:553)
at hudson.model.Run.run(Run.java:1390)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:145)