catch bad screen on timer

Author: willmcgugan

src/textual/demo/demo_app.py

@@ -65,6 +65,8 @@ class DemoApp(App):
     ]
 
     def action_maximize(self) -> None:
+        if self.screen.is_maximized:
+            return
         if self.screen.focused is None:
             self.notify(
                 "Nothing to be maximized (try pressing [b]tab[/b])",

src/textual/message_pump.py

@@ -836,6 +836,13 @@ def post_message(self, message: Message) -> bool:
     async def on_callback(self, event: events.Callback) -> None:
         if self.app._closing:
             return
+        try:
+            self.app.screen
+        except Exception:
+            self.log.warning(
+                f"Not invoking timer callback {event.callback!r} because there is no screen."
+            )
+            return
         await invoke(event.callback)
 
     async def on_timer(self, event: events.Timer) -> None: