Add debug complete event `None`-guard for when already reset

tractor/_debug.py

@@ -126,6 +126,7 @@ class Lock:
         try:
             # sometimes the ``trio`` might already be terminated in
             # which case this call will raise.
+            if cls.local_pdb_complete is not None:
                 cls.local_pdb_complete.set()
         finally:
             # restore original sigint handler