Fixed an iconTheme lerping problem with ChipThemeData. (#112216)

Author: darrenaustin

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

@@ -510,7 +510,9 @@ class ChipThemeData with Diagnosticable {
       brightness: t < 0.5 ? a?.brightness ?? Brightness.light : b?.brightness ?? Brightness.light,
       elevation: lerpDouble(a?.elevation, b?.elevation, t),
       pressElevation: lerpDouble(a?.pressElevation, b?.pressElevation, t),
-      iconTheme: IconThemeData.lerp(a?.iconTheme, b?.iconTheme, t),
+      iconTheme: a?.iconTheme != null || b?.iconTheme != null
+        ? IconThemeData.lerp(a?.iconTheme, b?.iconTheme, t)
+        : null,
     );
   }
 

packages/flutter/test/material/chip_theme_test.dart

@@ -443,6 +443,7 @@ void main() {
     expect(lerp.elevation, 3.0);
     expect(lerp.pressElevation, 7.0);
     expect(lerp.checkmarkColor, equals(middleGrey));
+    expect(lerp.iconTheme, isNull);
 
     expect(ChipThemeData.lerp(null, null, 0.25), isNull);
 
@@ -466,6 +467,7 @@ void main() {
     expect(lerpANull25.elevation, 1.25);
     expect(lerpANull25.pressElevation, 2.5);
     expect(lerpANull25.checkmarkColor, equals(Colors.white.withAlpha(0x40)));
+    expect(lerp.iconTheme, isNull);
 
     final ChipThemeData lerpANull75 = ChipThemeData.lerp(null, chipThemeWhite, 0.75)!;
     expect(lerpANull75.backgroundColor, equals(Colors.black.withAlpha(0x17)));
@@ -508,6 +510,7 @@ void main() {
     expect(lerpBNull25.elevation, 0.75);
     expect(lerpBNull25.pressElevation, 3.0);
     expect(lerpBNull25.checkmarkColor, equals(Colors.black.withAlpha(0xbf)));
+    expect(lerp.iconTheme, isNull);
 
     final ChipThemeData lerpBNull75 = ChipThemeData.lerp(chipThemeBlack, null, 0.75)!;
     expect(lerpBNull75.backgroundColor, equals(Colors.white.withAlpha(0x08)));
@@ -529,6 +532,7 @@ void main() {
     expect(lerpBNull75.elevation, 0.25);
     expect(lerpBNull75.pressElevation, 1.0);
     expect(lerpBNull75.checkmarkColor, equals(Colors.black.withAlpha(0x40)));
+    expect(lerp.iconTheme, isNull);
   });
 
   testWidgets('Chip uses stateful color from chip theme', (WidgetTester tester) async {