Why am I getting a "Debugger Update Job" error when running the debugger?

I run the debugger with error on the first attempt. On subsequent attempts the debugger pops up a "problem Occurred" dialog box that says;

Debugger Update Job has encountered a problem.

An internal error occurred during; "Debugger Update Job"

Opening the Details window shows:

An internal error occurred during; "Debugger Update Job"

java.lang.NullPointerException

I can still run the debugger OK.

I have no idea how to fix this. I have also tried to reproduce the error by running the debugger and then terminating without running the code and the error does not show. It only happens after the debugger has run the code and I exit the debugger and then come back into the debugger.

Anyone else seen this and how it was fixed?