Fix crash when editableText is not mounted (#171159)

Fix issue when text_selection crashes due to editableText not being
mounted.

No issue filled (internal b/425840577)

Likely introduced by https://github.com/flutter/flutter/pull/167881

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packages/flutter/lib/src/widgets/text_selection.dart

@@ -2933,7 +2933,8 @@ class TextSelectionGestureDetectorBuilder {
     _longPressStartedWithoutFocus = false;
     _dragStartViewportOffset = 0.0;
     _dragStartScrollOffset = 0.0;
-    if (defaultTargetPlatform == TargetPlatform.iOS &&
+    if (_isEditableTextMounted &&
+        defaultTargetPlatform == TargetPlatform.iOS &&
         delegate.selectionEnabled &&
         editableText.textEditingValue.selection.isCollapsed) {
       // Update the floating cursor.

packages/flutter/test/widgets/text_selection_test.dart

@@ -536,7 +536,6 @@ void main() {
     },
     variant: const TargetPlatformVariant(<TargetPlatform>{TargetPlatform.iOS}),
   );
-
   testWidgets(
     'test TextSelectionGestureDetectorBuilder long press on non-Apple Platforms',
     (WidgetTester tester) async {
@@ -557,6 +556,49 @@ void main() {
     ),
   );
 
+  testWidgets(
+    'does not crash when long press is cancelled after unmounting',
+    (WidgetTester tester) async {
+      // Regression test for b/425840577.
+      final ScrollController scrollController = ScrollController();
+      addTearDown(scrollController.dispose);
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Material(
+            child: CustomScrollView(
+              controller: scrollController,
+              slivers: <Widget>[
+                SliverList(
+                  delegate: SliverChildBuilderDelegate(
+                    (_, int index) => index == 0 ? const TextField() : const SizedBox(height: 50),
+                    childCount: 200,
+                    addAutomaticKeepAlives: false,
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+
+      final EditableTextState state = tester.state<EditableTextState>(find.byType(EditableText));
+      // Start a long press, don't release it, and don't completely reach kLongPressTimeout so the
+      // gesture is not accepted and is cancelled when the recognizer is disposed.
+      await tester.startGesture(tester.getCenter(find.byType(TextField)));
+      await tester.pump(const Duration(milliseconds: 200));
+      await tester.pumpAndSettle();
+
+      // While attempting to long press, scroll the TextField out of view
+      // to dispose of it and its gesture recognizers.
+      scrollController.jumpTo(8000.0);
+      await tester.pump();
+      expect(state.mounted, isFalse);
+      // Should reach the end of the test without any failures.
+    },
+    variant: TargetPlatformVariant.only(TargetPlatform.iOS),
+  );
+
   testWidgets(
     'TextSelectionGestureDetectorBuilder right click Apple platforms',
     (WidgetTester tester) async {