Changes according to review

Author: davidbrochart

nbclient/client.py

@@ -354,7 +354,7 @@ async def _async_cleanup_kernel(self) -> None:
         finally:
             # Remove any state left over even if we failed to stop the kernel
             await ensure_async(self.km.cleanup())
-            if self.kc is not None:
+            if getattr(self, "kc") and self.kc is not None:
                 await ensure_async(self.kc.stop_channels())
                 self.kc = None