[TestExitDuringStep] Adjust after a recent type map change.

[zturner]

lgtm, probably don't need to ask for review on this kind of patch :)

packages/Python/lldbsuite/test/functionalities/thread/exit_during_step/TestExitDuringStep.py

def exit_during_step_base(self, step_cmd, step_stop_reason, test_thread_state):
             self.assertTrue(thread2.IsStopped(), "Thread 2 didn't stop during breakpoint")
             self.assertTrue(thread3.IsStopped(), "Thread 3 didn't stop during breakpoint")
             return
 
         # Find the thread that is stopped at the breakpoint
         stepping_thread = None
         for thread in process:
             expected_bp_desc = "breakpoint %s." % self.bp_num
-            if expected_bp_desc in thread.GetStopDescription(100):
+            stop_desc = thread.GetStopDescription(100)
+            if stop_desc and (expected_bp_desc in stop_desc):
                 stepping_thread = thread
                 break
         self.assertTrue(stepping_thread != None, "unable to find thread stopped at %s" % expected_bp_desc)
 
         current_line = self.breakpoint
         stepping_frame = stepping_thread.GetFrameAtIndex(0)
         self.assertTrue(current_line == stepping_frame.GetLineEntry().GetLine(), "Starting line for stepping doesn't match breakpoint line.")