See the stacktrace below if sbt-launch.jar is not executable. I upgraded to SBT 0.13.2 installing it RPM and it so happens that the jar is only "a+r".
Once I did a "chmod a+x sbt-launch.jar" the NPE did not happen anymore.

java.lang.NullPointerException
at org.jvnet.hudson.plugins.SbtPluginBuilder.buildCmdLine(SbtPluginBuilder.java:159)
at org.jvnet.hudson.plugins.SbtPluginBuilder.perform(SbtPluginBuilder.java:111)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:740)
at hudson.model.Build$BuildExecution.build(Build.java:198)
at hudson.model.Build$BuildExecution.doRun(Build.java:159)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:519)
at hudson.model.Run.execute(Run.java:1703)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:231)