Fix Slider throws an error when _labelPainter text is null (flutter#148462)

*Fix flutter#148159*

Author: hello-coder-xu

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

@@ -1720,7 +1720,7 @@ class _RenderSlider extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
     if (isInteractive && label != null && !_valueIndicatorAnimation.isDismissed) {
       if (showValueIndicator) {
         _state.paintValueIndicator = (PaintingContext context, Offset offset) {
-          if (attached) {
+          if (attached && _labelPainter.text != null) {
             _sliderTheme.valueIndicatorShape!.paint(
               context,
               offset + thumbCenter,

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

@@ -3485,7 +3485,6 @@ class _DropSliderValueIndicatorPathPainter {
       return;
     }
     assert(!sizeWithOverflow.isEmpty);
-
     final double rectangleWidth = _upperRectangleWidth(labelPainter, scale);
     final double horizontalShift = getHorizontalShift(
       parentBox: parentBox,

packages/flutter/test/material/slider_test.dart

@@ -4384,4 +4384,47 @@ void main() {
     await gesture.moveBy(const Offset(1.0, 0.0));
     expect(onChangeCallbackCount, 1);
   });
+
+  testWidgets('Skip drawing ValueIndicator shape when label painter text is null', (WidgetTester tester) async {
+    double sliderValue = 10;
+
+    await tester.pumpWidget(
+        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;
+                    });
+                  },
+                ),
+              );
+            },
+          ),
+        ),
+      );
+
+      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, 2));
+
+      // Tap on the 75% position of the Slider.
+      await tester.tapAt(tapPositionRight);
+      await tester.pumpAndSettle();
+      expect(valueIndicatorBox, paintsExactlyCountTimes(#drawPath, 1));
+  });
 }