Fix tracing_middleware reference errors

Author: ekampf

graphql/execution/executor.py

@@ -39,6 +39,7 @@ def execute(schema, document_ast, root_value=None, context_value=None,
         'not multiple versions of GraphQL installed in your node_modules directory.'
     )
 
+    tracing_middleware = None
     if tracing:
         tracing_middleware = TracingMiddleware()
         tracing_middleware.start()
@@ -83,8 +84,10 @@ def on_resolve(data):
         if isinstance(data, Observable):
             return data
 
-        tracing_middleware.end()
-        extensions = dict(tracing=tracing_middleware.tracing_dict)
+        extensions = dict()
+        if tracing_middleware:
+            tracing_middleware.end()
+            extensions['tracing'] = tracing_middleware.tracing_dict
 
         if not context.errors:
             return ExecutionResult(data=data, extensions=extensions)