diff --git a/CHANGELOG.md b/CHANGELOG.md
index dc7242d4ed..8cd6cba6e2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 
+- Fix traceback on exit https://github.com/Textualize/textual/pull/4575
 - Fixed `Markdown.goto_anchor` no longer scrolling the heading into view https://github.com/Textualize/textual/pull/4583
 
 ## [0.63.6] - 2024-05-29
diff --git a/src/textual/app.py b/src/textual/app.py
index a0b914a9ea..ccbe85d3cc 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -111,6 +111,7 @@
     _SystemModalScreen,
 )
 from .signal import Signal
+from .timer import Timer
 from .widget import AwaitMount, Widget
 from .widgets._toast import ToastRack
 from .worker import NoActiveWorker, get_current_worker
@@ -2522,8 +2523,7 @@ async def invoke_ready_callback() -> None:
                 try:
                     await self.animator.stop()
                 finally:
-                    for timer in list(self._timers):
-                        timer.stop()
+                    await Timer._stop_all(self._timers)
 
         self._running = True
         try:
@@ -3373,6 +3373,7 @@ async def _prune_node(self, root: Widget) -> None:
             root: Node to remove.
         """
         # Pruning a node that has been removed is a no-op
+
         if root not in self._registry:
             return
 
diff --git a/src/textual/message_pump.py b/src/textual/message_pump.py
index 8b7fd18dca..b13536aeed 100644
--- a/src/textual/message_pump.py
+++ b/src/textual/message_pump.py
@@ -247,8 +247,9 @@ def app(self) -> "App[object]":
     @property
     def is_attached(self) -> bool:
         """Is this node linked to the app through the DOM?"""
+        if self.is_dom_root:
+            return True
         node: MessagePump | None = self
-
         while (node := node._parent) is not None:
             if node.is_dom_root:
                 return True
@@ -479,10 +480,9 @@ async def _close_messages(self, wait: bool = True) -> None:
         if self._closed or self._closing:
             return
         self._closing = True
-        stop_timers = list(self._timers)
-        for timer in stop_timers:
-            timer.stop()
-        self._timers.clear()
+        if self._timers:
+            await Timer._stop_all(self._timers)
+            self._timers.clear()
         await self._message_queue.put(events.Unmount())
         Reactive._reset_object(self)
         await self._message_queue.put(None)
@@ -519,8 +519,9 @@ async def _process_messages(self) -> None:
             pass
         finally:
             self._running = False
-            for timer in list(self._timers):
-                timer.stop()
+            if self._timers:
+                await Timer._stop_all(self._timers)
+                self._timers.clear()
 
     async def _pre_process(self) -> bool:
         """Procedure to run before processing messages.
@@ -849,6 +850,8 @@ def get_key_handler(pump: MessagePump, key: str) -> Callable | None:
         return handled
 
     async def on_timer(self, event: events.Timer) -> None:
+        if not self.app._running:
+            return
         event.prevent_default()
         event.stop()
         if event.callback is not None:
diff --git a/src/textual/reactive.py b/src/textual/reactive.py
index bab60a971a..c9b153a743 100644
--- a/src/textual/reactive.py
+++ b/src/textual/reactive.py
@@ -73,6 +73,7 @@ def invoke_watcher(
         value: The new value of the attribute.
     """
     _rich_traceback_omit = True
+
     param_count = count_parameters(watch_function)
     reset_token = active_message_pump.set(watcher_object)
     try:
diff --git a/src/textual/signal.py b/src/textual/signal.py
index 8fe5d43a23..7128b5530b 100644
--- a/src/textual/signal.py
+++ b/src/textual/signal.py
@@ -109,7 +109,7 @@ def publish(self, data: SignalT) -> None:
         """
 
         for node, callbacks in list(self._subscriptions.items()):
-            if not node.is_running:
+            if not (node.is_running and node.is_attached):
                 # Removed nodes that are no longer running
                 self._subscriptions.pop(node)
             else:
diff --git a/src/textual/timer.py b/src/textual/timer.py
index bf086262d0..6cc664369f 100644
--- a/src/textual/timer.py
+++ b/src/textual/timer.py
@@ -7,8 +7,8 @@
 from __future__ import annotations
 
 import weakref
-from asyncio import CancelledError, Event, Task, create_task
-from typing import Any, Awaitable, Callable, Union
+from asyncio import CancelledError, Event, Task, create_task, gather
+from typing import Any, Awaitable, Callable, Iterable, Union
 
 from rich.repr import Result, rich_repr
 
@@ -83,12 +83,44 @@ def _start(self) -> None:
         """Start the timer."""
         self._task = create_task(self._run_timer(), name=self.name)
 
-    def stop(self) -> None:
-        """Stop the timer."""
-        if self._task is not None:
-            self._active.set()
-            self._task.cancel()
-            self._task = None
+    def stop(self) -> Task:
+        """Stop the timer.
+
+        Returns:
+            A Task object. Await this to wait until the timer has completed.
+
+        """
+        if self._task is None:
+
+            async def noop() -> None:
+                """A dummy task."""
+
+            return create_task(noop())
+
+        self._active.set()
+        self._task.cancel()
+        return self._task
+
+    @classmethod
+    async def _stop_all(cls, timers: Iterable[Timer]) -> None:
+        """Stop a number of timers, and await their completion.
+
+        Args:
+            timers: A number of timers.
+        """
+
+        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()
+                await timer._task
+
+        await gather(*[stop_timer(timer) for timer in list(timers)])
 
     def pause(self) -> None:
         """Pause the timer.
diff --git a/src/textual/widget.py b/src/textual/widget.py
index 49f6afa534..e5de1959e5 100644
--- a/src/textual/widget.py
+++ b/src/textual/widget.py
@@ -934,15 +934,16 @@ def mount(
             Only one of ``before`` or ``after`` can be provided. If both are
             provided a ``MountError`` will be raised.
         """
+        if self._closing:
+            return AwaitMount(self, [])
         if not self.is_attached:
             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]
-        unique_ids = set(ids_to_mount)
-        num_unique_ids = len(unique_ids)
-        num_widgets_with_ids = len(ids_to_mount)
-        if num_unique_ids != num_widgets_with_ids:
-            counter = Counter(widget.id for widget in widgets)
+        ids_to_mount = [
+            widget_id for widget in widgets if (widget_id := widget.id) is not None
+        ]
+        if len(set(ids_to_mount)) != len(ids_to_mount):
+            counter = Counter(ids_to_mount)
             for widget_id, count in counter.items():
                 if count > 1:
                     raise MountError(
@@ -3728,7 +3729,8 @@ async def mount_composed_widgets(self, widgets: list[Widget]) -> None:
         Args:
             widgets: A list of child widgets.
         """
-        await self.mount_all(widgets)
+        if widgets:
+            await self.mount_all(widgets)
 
     def _extend_compose(self, widgets: list[Widget]) -> None:
         """Hook to extend composed widgets.