fix a [SelectableRegion] crash bug (#120076)

* fix a crash bug

* review feedback

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

@@ -1771,11 +1771,11 @@ abstract class MultiSelectableSelectionContainerDelegate extends SelectionContai
     LayerLink? effectiveStartHandle = _startHandleLayer;
     LayerLink? effectiveEndHandle = _endHandleLayer;
     if (effectiveStartHandle != null || effectiveEndHandle != null) {
-      final Rect drawableArea = Rect
+      final Rect? drawableArea = hasSize ? Rect
         .fromLTWH(0, 0, containerSize.width, containerSize.height)
-        .inflate(_kSelectionHandleDrawableAreaPadding);
-      final bool hideStartHandle = value.startSelectionPoint == null || !drawableArea.contains(value.startSelectionPoint!.localPosition);
-      final bool hideEndHandle = value.endSelectionPoint == null || !drawableArea.contains(value.endSelectionPoint!.localPosition);
+        .inflate(_kSelectionHandleDrawableAreaPadding) : null;
+      final bool hideStartHandle = value.startSelectionPoint == null || drawableArea ==  null || !drawableArea.contains(value.startSelectionPoint!.localPosition);
+      final bool hideEndHandle = value.endSelectionPoint == null || drawableArea ==  null|| !drawableArea.contains(value.endSelectionPoint!.localPosition);
       effectiveStartHandle = hideStartHandle ? null : _startHandleLayer;
       effectiveEndHandle = hideEndHandle ? null : _endHandleLayer;
     }

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

@@ -298,12 +298,26 @@ abstract class SelectionContainerDelegate implements SelectionHandler, Selection
     return box.getTransformTo(ancestor);
   }
 
+  /// Whether the [SelectionContainer] has undergone layout and has a size.
+  ///
+  /// See also:
+  ///
+  ///  * [RenderBox.hasSize], which is used internally by this method.
+  bool get hasSize {
+    assert(
+    _selectionContainerContext?.findRenderObject() != null,
+    'The _selectionContainerContext must have a renderObject, such as after the first build has completed.',
+    );
+    final RenderBox box = _selectionContainerContext!.findRenderObject()! as RenderBox;
+    return box.hasSize;
+  }
+
   /// Gets the size of the [SelectionContainer] of this delegate.
   ///
   /// Can only be called after [SelectionContainer] is laid out.
   Size get containerSize {
     assert(
-      _selectionContainerContext?.findRenderObject() != null,
+      hasSize,
       'containerSize cannot be called before SelectionContainer is laid out.',
     );
     final RenderBox box = _selectionContainerContext!.findRenderObject()! as RenderBox;

packages/flutter/test/widgets/selectable_region_test.dart

@@ -73,6 +73,37 @@ void main() {
       await gesture.up();
     }, skip: kIsWeb); // https://github.com/flutter/flutter/issues/102410.
 
+    testWidgets('Does not crash when using Navigator pages', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/119776
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Navigator(
+            pages: <Page<void>> [
+              MaterialPage<void>(
+                child: Column(
+                  children: <Widget>[
+                    const Text('How are you?'),
+                    SelectableRegion(
+                      focusNode: FocusNode(),
+                      selectionControls: materialTextSelectionControls,
+                      child: const SelectAllWidget(child: SizedBox(width: 100, height: 100)),
+                    ),
+                    const Text('Fine, thank you.'),
+                  ],
+                ),
+              ),
+              const MaterialPage<void>(
+                child: Scaffold(body: Text('Foreground Page')),
+              ),
+            ],
+            onPopPage: (_, __) => false,
+          ),
+        ),
+      );
+
+      expect(tester.takeException(), isNull);
+    });
+
     testWidgets('can draw handles when they are at rect boundaries', (WidgetTester tester) async {
       final UniqueKey spy = UniqueKey();
       await tester.pumpWidget(