Fix a crash when setting a kaymap before app is mounted

Fixes #5742.

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## Unreleased
+
+### Fixed
+
+- Fixed a crash when setting keymap before app mount https://github.com/Textualize/textual/issues/5742
+
 ## [3.1.0] - 2025-04-12
 
 ### Fixed

src/textual/dom.py

@@ -1818,7 +1818,8 @@ class DOMNode(MessagePump):
         See [actions](/guide/actions#dynamic-actions) for how to use this method.
 
         """
-        self.screen.refresh_bindings()
+        if self._is_mounted:
+            self.screen.refresh_bindings()
 
     async def action_toggle(self, attribute_name: str) -> None:
         """Toggle an attribute on the node.

tests/test_keymap.py

@@ -192,3 +192,28 @@ async def test_keymap_child_with_different_id_overridden():
         await pilot.press("i")
         assert parent_counter == 1
         assert child_counter == 1
+
+
+async def test_set_keymap_before_app_mount():
+    """Ensure we can set the keymap before mount without crash.
+
+    https://github.com/Textualize/textual/issues/5742
+    """
+
+    worked = False
+
+    class MyApp(App[None]):
+
+        BINDINGS = [Binding(key="x", action="test", id="test")]
+
+        def __init__(self) -> None:
+            super().__init__()
+            self.update_keymap({"test": "y"})
+
+        def action_test(self) -> None:
+            nonlocal worked
+            worked = True
+
+    async with MyApp().run_test() as pilot:
+        await pilot.press("y")
+        assert worked is True