Check if stub is available before trying to fetch it

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/client/electricity_trading/_client.py

@@ -177,7 +177,7 @@ def stub(self) -> ElectricityTradingServiceAsyncStub:
         Raises:
             ClientNotConnected: If the client is not connected to the server.
         """
-        if self._channel is None:
+        if self._stub is None:
             raise ClientNotConnected(server_url=self.server_url, operation="stub")
         return self._stub