avoid KeyError in handle_disconnect (fixes #4304)

Author: falkoschindler

nicegui/client.py

@@ -257,6 +257,8 @@ def handle_disconnect(self, socket_id: str) -> None:
         In contrast to connect handlers, disconnect handlers are not called during a reconnect.
         This behavior should be fixed in version 3.0.
         """
+        if socket_id not in self._socket_to_document_id:
+            return
         document_id = self._socket_to_document_id.pop(socket_id)
         self._cancel_delete_task(document_id)
         self._num_connections[document_id] -= 1