avoid a nullpointer bug when the stack is null

This is a simple fix to avoid a nullpointer that had occurred in some 1.7 jdk bytecode situation.  There is obviously a bug in the bytecodeparser lib as well where enterMethod is not called but exitMethod is called.  I am not sure how to fix that bug, but we can avoid it easily enough so we can at least get basic play apps working.....in our case the below was throwing a nullpointer when calling the first method in a catch block for some reason.