restore sanity check

src/textual/app.py

@@ -3362,10 +3362,11 @@ async def _prune_nodes(self, widgets: list[Widget]) -> None:
         Args:
             widgets: Widgets to remove.
         """
+        if not self.is_attached:
+            return
         async with self._dom_lock:
             for widget in widgets:
-                if self.is_attached:
-                    await self._prune_node(widget)
+                await self._prune_node(widget)
 
     async def _prune_node(self, root: Widget) -> None:
         """Remove a node and its children. Children are removed before parents.

src/textual/timer.py

@@ -104,7 +104,12 @@ async def _stop_all(cls, timers: Iterable[Timer]) -> None:
             timers: A number of timers.
         """
 
-        async def stop_timer(timer) -> None:
+        async def stop_timer(timer: Timer) -> None:
+            """Stop a timer and wait for it to finish.
+
+            Args:
+                timer: A Timer instance.
+            """
             if timer._task is not None:
                 timer._active.set()
                 timer._task.cancel()

src/textual/widget.py

@@ -935,7 +935,6 @@ def mount(
             provided a ``MountError`` will be raised.
         """
         if not self.is_attached:
-            return AwaitMount(self, [])
             raise MountError(f"Can't mount widget(s) before {self!r} is mounted")
         # Check for duplicate IDs in the incoming widgets
         ids_to_mount = [widget.id for widget in widgets if widget.id is not None]