Merge pull request #1363 from davep/prune-unused-mounterror-import

Remove unused import of MounrError in app.py

Author: davep

src/textual/app.py

@@ -63,7 +63,7 @@
 from .reactive import Reactive
 from .renderables.blank import Blank
 from .screen import Screen
-from .widget import AwaitMount, MountError, Widget
+from .widget import AwaitMount, Widget
 
 
 if TYPE_CHECKING: