🍒backtracegenerator: don't crash when the debugger crashes

when the debugger crashes slotOnErrorOccurred runs before slotProcessExited and causes the latter to trip over the since-nullpointered members. instead guard the delete operations against nullptrs. this is done in both functions to increase reliablity even though technically it's only needed in slotProcessExited

BUG: 485669 (cherry picked from commit 8f4a57a6)