Fix code sample failing in smoke test (#144709)

This fixes the silent failure reported in https://github.com/flutter/flutter/issues/144353. I am experimenting in https://github.com/flutter/flutter/pull/144706 with whether the failure should have been non-silent.

examples/api/lib/material/color_scheme/dynamic_content_color.0.dart

@@ -188,6 +188,9 @@ class _DynamicColorExampleState extends State<DynamicColorExample> {
   Future<void> _updateImage(ImageProvider provider) async {
     final ColorScheme newColorScheme = await ColorScheme.fromImageProvider(
         provider: provider, brightness: isLight ? Brightness.light : Brightness.dark);
+    if (!mounted) {
+      return;
+    }
     setState(() {
       selectedImage = widget.images.indexOf(provider);
       currentColorScheme = newColorScheme;