interface: check if future already done in handle_disconnect

future could get cancelled in network.py in which case set_result raised

electrum/interface.py

@@ -336,6 +336,7 @@ class Interface(Logger):
                 self.logger.debug(f"(disconnect) trace for {repr(e)}", exc_info=True)
             finally:
                 await self.network.connection_down(self)
+                if not self.got_disconnected.done():
                     self.got_disconnected.set_result(1)
                 # if was not 'ready' yet, schedule waiting coroutines:
                 self.ready.cancel()