A null check needs to be wrapped around CheckoutTask.java:207.

if (buildProject.equals(""))

{ buildProject = null; }

Should be:

if ((buildProject != null) && buildProject.equals("")) { buildProject = null; }

I was able to cause this NPE by using Job DSL and not entering a value for buildProject (or including buildProject()). This caused config.xml to not have an entry for buildProject and caused this variable to be null at this point.

I was able to get past this by specifying buildProject("") in my Job DSL script. It seems like the code should be made more robust here to handle this case better.