Raise when trying to create a receiver while disconnected

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/dispatch/_bg_service.py

@@ -186,7 +186,12 @@ async def new_running_state_event_receiver(
 
         Returns:
             A new receiver for running state status.
+
+        Raises:
+            RuntimeError: If the dispatch service is not running.
         """
+        if not self.is_running:
+            raise RuntimeError("Dispatch service not started")
 
         # Wait with timeout for the initial fetch to complete
         await asyncio.wait_for(