Fix assertion error when transport is already closed when _receive is called

Author: leszekhanusz

gql/transport/websockets.py

@@ -175,8 +175,9 @@ async def _receive(self) -> str:
         """Wait the next message from the websocket connection and log the answer
         """
 
-        # We should always have an active websocket connection here
-        assert self.websocket is not None
+        # It is possible that the websocket has been already closed in another task
+        if self.websocket is None:
+            raise TransportClosed("Transport is already closed")
 
         # Wait for the next websocket frame. Can raise ConnectionClosed
         data: Data = await self.websocket.recv()
@@ -387,6 +388,8 @@ async def _receive_data_loop(self) -> None:
                 except (ConnectionClosed, TransportProtocolError) as e:
                     await self._fail(e, clean_close=False)
                     break
+                except TransportClosed:
+                    break
 
                 # Parse the answer
                 try: