fix slider text null error

Author: hello-coder-xu

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

@@ -3485,7 +3485,10 @@ class _DropSliderValueIndicatorPathPainter {
       return;
     }
     assert(!sizeWithOverflow.isEmpty);
-
+    if (labelPainter.text == null) {
+      // labelPainter.text is null do not draw anything, so it's safe to just return.
+      return;
+    }
     final double rectangleWidth = _upperRectangleWidth(labelPainter, scale);
     final double horizontalShift = getHorizontalShift(
       parentBox: parentBox,

packages/flutter/test/material/slider_theme_test.dart

@@ -2648,6 +2648,35 @@ void main() {
       }
     });
   });
+  testWidgets('indicator shape skips all painting at label is null',
+      (WidgetTester tester) async {
+    debugDisableShadows = false;
+    try {
+      await tester.pumpWidget(_buildSilderApp());
+
+      final RenderBox valueIndicatorBox =
+          tester.renderObject(find.byType(Overlay));
+
+      // Calculate a specific position on the Slider
+      final Rect sliderRect = tester.getRect(find.byType(Slider));
+      final Offset tapPositionLeft = Offset(
+          sliderRect.left + sliderRect.width * 0.25, sliderRect.center.dy);
+      final Offset tapPositionRight = Offset(
+          sliderRect.left + sliderRect.width * 0.75, sliderRect.center.dy);
+
+      // Tap on the 25% position of the Slider
+      await tester.tapAt(tapPositionLeft);
+      await tester.pumpAndSettle();
+      expect(valueIndicatorBox, paintsExactlyCountTimes(#drawPath, 1));
+
+      // Tap on the 75% position of the Slider
+      await tester.tapAt(tapPositionRight);
+      await tester.pumpAndSettle();
+      expect(valueIndicatorBox, paintsExactlyCountTimes(#drawPath, 0));
+    } finally {
+      debugDisableShadows = true;
+    }
+  });
 }
 
 class RoundedRectSliderTrackShapeWithCustomAdditionalActiveTrackHeight extends RoundedRectSliderTrackShape {
@@ -2722,3 +2751,25 @@ Widget _buildRangeApp(
     ),
   );
 }
+Widget _buildSilderApp() {
+  double sliderValue = 10;
+  return MaterialApp(
+    home: StatefulBuilder(
+      builder: (BuildContext context, void Function(void Function()) setState) {
+        return Material(
+          child: Slider(
+            value: sliderValue,
+            max: 100,
+            label: sliderValue > 50 ? null : sliderValue.toString(),
+            divisions: 10,
+            onChanged: (double value) {
+              setState(() {
+                sliderValue = value;
+              });
+            },
+          ),
+        );
+      },
+    ),
+  );
+}