Review #242194: NPE if attach debugger

Please review the fix of bug 
https://netbeans.org/bugzilla/show_bug.cgi?id=242194

Description:
A NPE is thrown occasionally during debugger attach.

Solution:
There is a missing condition on currentThread, which can be null. A 
condition for null is added as a fix.

Impact assesment:
Functionality of debugger attach

Risk:
Very low

Thanks,
Martin