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

@@ -184,7 +184,13 @@ 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._tasks:
+            raise RuntimeError("Dispatch service not started")
+
         # Find all matching dispatches based on the type and collect them
         dispatches = [
             dispatch for dispatch in self._dispatches.values() if dispatch.type == type