CircularProgressIndicator throws null exception (#163356)

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This PR adds a null check before accessing value, preventing errors
caused by the null check operator used on null value.

Fixes #163253 

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

@@ -606,7 +606,7 @@ class _CircularProgressIndicatorPainter extends CustomPainter {
             ..strokeCap = strokeCap ?? StrokeCap.round
             ..style = PaintingStyle.stroke;
       // If hasGap is true, draw the background arc with a gap.
-      if (hasGap && value! > _epsilon) {
+      if (hasGap && value != null && value! > _epsilon) {
         final double arcRadius = arcActualSize.shortestSide / 2;
         final double strokeRadius = strokeWidth / arcRadius;
         final double gapRadius = trackGap! / arcRadius;

packages/flutter/test/material/progress_indicator_theme_test.dart

@@ -376,6 +376,25 @@ void main() {
     );
   });
 
+  testWidgets(
+    'CircularProgressIndicator.year2023 set to false and provided circularTrackColor does not throw exception',
+    (WidgetTester tester) async {
+      const Color circularTrackColor = Color(0XFF0000FF);
+      final ThemeData theme = ThemeData(
+        progressIndicatorTheme: const ProgressIndicatorThemeData(
+          circularTrackColor: circularTrackColor,
+          year2023: false,
+        ),
+      );
+
+      await tester.pumpWidget(
+        Theme(data: theme, child: const Center(child: CircularProgressIndicator())),
+      );
+
+      expect(tester.takeException(), null);
+    },
+  );
+
   testWidgets(
     'Opt into 2024 CircularProgressIndicator appearance with ProgressIndicatorThemeData.year2023',
     (WidgetTester tester) async {