Complete the Scene.toImage future with an error if rasterization fails (#24877)

Also fix the nullability annotations on _futurize

Fixes flutter/flutter#76591

Author: jason-simmons

lib/ui/compositing.dart

@@ -31,13 +31,17 @@ class Scene extends NativeFieldWrapperClass2 {
     if (width <= 0 || height <= 0) {
       throw Exception('Invalid image dimensions.');
     }
-    return _futurize((_Callback<Image> callback) => _toImage(width, height, (_Image image) {
-        callback(Image._(image));
+    return _futurize((_Callback<Image?> callback) => _toImage(width, height, (_Image? image) {
+        if (image == null) {
+          callback(null);
+        } else {
+          callback(Image._(image));
+        }
       }),
     );
   }
 
-  String? _toImage(int width, int height, _Callback<_Image> callback) native 'Scene_toImage';
+  String? _toImage(int width, int height, _Callback<_Image?> callback) native 'Scene_toImage';
 
   /// Releases the resources used by this scene.
   ///

lib/ui/painting.dart

@@ -5273,7 +5273,7 @@ typedef _Callback<T> = void Function(T result);
 ///
 /// Return value should be null on success, and a string error message on
 /// failure.
-typedef _Callbacker<T> = String? Function(_Callback<T> callback);
+typedef _Callbacker<T> = String? Function(_Callback<T?> callback);
 
 /// Converts a method that receives a value-returning callback to a method that
 /// returns a Future.
@@ -5298,7 +5298,7 @@ typedef _Callbacker<T> = String? Function(_Callback<T> callback);
 /// ```
 Future<T> _futurize<T>(_Callbacker<T> callbacker) {
   final Completer<T> completer = Completer<T>.sync();
-  final String? error = callbacker((T t) {
+  final String? error = callbacker((T? t) {
     if (t == null) {
       completer.completeError(Exception('operation failed'));
     } else {