Fix `RangeSlider` throws a null-check error after `clearSemantics` is called (#141965)

fixes [Null-check operator on RangeSlider's _startSemanticsNode](https://github.com/flutter/flutter/issues/141953)

packages/flutter/lib/src/material/range_slider.dart

@@ -1628,10 +1628,10 @@ class _RenderRangeSlider extends RenderBox with RelayoutWhenSystemFontsChangeMix
   }
 
   /// Describe the semantics of the start thumb.
-  SemanticsNode? _startSemanticsNode = SemanticsNode();
+  SemanticsNode? _startSemanticsNode;
 
   /// Describe the semantics of the end thumb.
-  SemanticsNode? _endSemanticsNode = SemanticsNode();
+  SemanticsNode? _endSemanticsNode;
 
   // Create the semantics configuration for a single value.
   SemanticsConfiguration _createSemanticsConfiguration(
@@ -1697,6 +1697,10 @@ class _RenderRangeSlider extends RenderBox with RelayoutWhenSystemFontsChangeMix
       width: kMinInteractiveDimension,
       height: kMinInteractiveDimension,
     );
+
+    _startSemanticsNode ??= SemanticsNode();
+    _endSemanticsNode ??= SemanticsNode();
+
     switch (textDirection) {
       case TextDirection.ltr:
         _startSemanticsNode!.rect = leftRect;

packages/flutter/test/material/range_slider_test.dart

@@ -9,6 +9,8 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/src/physics/utils.dart' show nearEqual;
 import 'package:flutter_test/flutter_test.dart';
 
+import '../widgets/semantics_tester.dart';
+
 void main() {
   // Regression test for https://github.com/flutter/flutter/issues/105833
   testWidgets('Drag gesture uses provided gesture settings', (WidgetTester tester) async {
@@ -2610,4 +2612,36 @@ void main() {
     // No exception should be thrown.
     expect(tester.takeException(), null);
   });
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/141953.
+  testWidgets('Semantic nodes do not throw an error after clearSemantics', (WidgetTester tester) async {
+    SemanticsTester semantics = SemanticsTester(tester);
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Material(
+          child: RangeSlider(
+            values: const RangeValues(40, 80),
+            max: 100,
+            onChanged: (RangeValues newValue) { },
+          ),
+        ),
+      ),
+    );
+
+    // Dispose the semantics to trigger clearSemantics.
+    semantics.dispose();
+    await tester.pumpAndSettle();
+
+    expect(tester.takeException(), isNull);
+
+    // Initialize the semantics again.
+    semantics = SemanticsTester(tester);
+    await tester.pumpAndSettle();
+
+    expect(tester.takeException(), isNull);
+
+    semantics.dispose();
+  }, semanticsEnabled: false);
 }