catch no screen

Author: willmcgugan

src/textual/message_pump.py

@@ -844,6 +844,13 @@ async def on_timer(self, event: events.Timer) -> None:
         event.prevent_default()
         event.stop()
         if event.callback is not None:
+            try:
+                self.app.screen
+            except Exception:
+                self.log.warning(
+                    f"Not invoking timer callback {event.callback!r} because there is no screen."
+                )
+                return
             try:
                 await invoke(event.callback)
             except Exception as error: