Some uninitialized build triggers may not yet have a write token.

endpoints/api/trigger.py

@@ -81,7 +81,9 @@ class BuildTrigger(RepositoryParamResource):
                   'service': trigger.service.name, 'config': config_dict},
                  repo=model.get_repository(namespace, repository))
 
-    trigger.write_token.delete_instance()
+    if trigger.write_token is not None:
+      trigger.write_token.delete_instance()
+
     trigger.delete_instance(recursive=True)
     return 'No Content', 204