How to resolve groovy script failure: Cannot invoke method getProjectByName() on null object

Not sure if you have access to the workspace as such in the testrunner.

I might be wrong but I've always run the test runner with an explicit project in mind. But the error you get tells me that workspace is null.

Right here:
testRunner.testCase.testSuite.project.workspace.getProjectByName("UserRights")

Consider why you need to separate a run into several different projects. It feels a little awkward tbh. Rather out that functionality in another (perhaps disabled) testsuite.