add a guard

Author: sentrivana

sentry_sdk/traces.py

@@ -451,6 +451,9 @@ def _end(self, end_timestamp: "Optional[Union[float, datetime]]" = None) -> None
         if self._scope is None:
             return
 
+        if not hasattr(self._previous_span_on_scope):
+            return
+
         with capture_internal_exceptions():
             old_span = self._previous_span_on_scope
             del self._previous_span_on_scope