Ensure the active message pump is set when app is the sender (#2302)

See https://github.com/Textualize/textual/issues/2301

CHANGELOG.md

@@ -28,6 +28,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Fixed order styles are applied in DataTable - allows combining of renderable styles and component classes https://github.com/Textualize/textual/pull/2272
 - Fix empty ListView preventing bindings from firing https://github.com/Textualize/textual/pull/2281
+- Fixed `active_message_pump.get` sometimes resulting in a `LookupError` https://github.com/Textualize/textual/issues/2301
 
 
 ## [0.19.1] - 2023-04-10

src/textual/app.py

@@ -1603,6 +1603,7 @@ class App(Generic[ReturnType], DOMNode):
         terminal_size: tuple[int, int] | None = None,
     ) -> None:
         self._set_active()
+        active_message_pump.set(self)
 
         if self.devtools is not None:
             from .devtools.client import DevtoolsConnectionError