fix an error about image cache

Samples/UIWidgetsSamples_2019_4/Assets/UIWidgetsGallery/demo/cupertino/cupertino_navigation_demo.cs

                                                         decoration: new BoxDecoration(
                                                             image: new DecorationImage(
                                                                 image: new FileImage(
-                                                                    file:""
+                                                                    file:"test.png"
                                                                 )
                                                             ),
                                                             shape: BoxShape.circle
                                                         decoration: new BoxDecoration(
                                                             image: new DecorationImage(
                                                                 image: new FileImage(
-                                                                    "people/square/sandra"
+                                                                    "test.png"
                                                                 )
                                                             ),
                                                             shape: BoxShape.circle

com.unity.uiwidgets/Runtime/painting/image_cache.cs

             }
 
             if (_cache.TryGetValue(key, out var image)) {
-                _lruKeys.Remove(image.node);
+                if (image.node != null) {
+                    _lruKeys.Remove(image.node);
+                }
                 _trackLiveImage(key, new _LiveImage(image.completer, image.sizeBytes, () => _liveImages.Remove(key)));
                 image.node = _lruKeys.AddLast(key);
                 return image.completer;

com.unity.uiwidgets/Runtime/painting/image_provider.cs

         }
 
         Future<Codec> _loadAsync(FileImage key, DecoderCallback decode) {
-            byte[] bytes = File.ReadAllBytes(Path.Combine(Application.streamingAssetsPath, key.file));
-            if (bytes != null && bytes.Length > 0) {
+            byte[] bytes = File.ReadAllBytes(Path.Combine(Application.dataPath, key.file));
+            if (bytes.Length > 0) {
                 return decode(bytes);
             }