update getting album thumbnail service and AlbumThumbnailProvider to show all grey image when album is empty

lib/photo_gallery.dart

@@ -85,7 +85,7 @@ class PhotoGallery {
   }
 
   /// Get album thumbnail by album id
-  static Future<List<int>> getAlbumThumbnail({
+  static Future<List<int>?> getAlbumThumbnail({
     required String albumId,
     MediumType? mediumType,
     int? width,
@@ -99,8 +99,7 @@ class PhotoGallery {
       'height': height,
       'highQuality': highQuality,
     });
-    if (bytes == null) throw "Failed to fetch thumbnail of album $albumId";
-    return new List<int>.from(bytes);
+    return bytes != null ? new List<int>.from(bytes) : null;
   }
 
   /// get medium file by medium id

lib/src/image_providers/album_thumbnail_provider.dart

@@ -17,7 +17,7 @@ class AlbumThumbnailProvider extends ImageProvider<AlbumThumbnailProvider> {
   final bool? highQuality;
 
   @override
-  ImageStreamCompleter load(key, decode) {
+  ImageStreamCompleter loadBuffer(key, decode) {
     return MultiFrameImageStreamCompleter(
       codec: _loadAsync(key, decode),
       scale: 1.0,
@@ -27,17 +27,22 @@ class AlbumThumbnailProvider extends ImageProvider<AlbumThumbnailProvider> {
     );
   }
 
-  Future<ui.Codec> _loadAsync(
-      AlbumThumbnailProvider key, DecoderCallback decode) async {
+  Future<ui.Codec> _loadAsync(AlbumThumbnailProvider key, DecoderBufferCallback decode) async {
     assert(key == this);
-    final bytes = await PhotoGallery.getAlbumThumbnail(
+    final data = await PhotoGallery.getAlbumThumbnail(
       albumId: albumId,
       mediumType: mediumType,
       height: height,
       width: width,
       highQuality: highQuality,
     );
-    return await decode(Uint8List.fromList(bytes));
+    ui.ImmutableBuffer buffer;
+    if (data != null) {
+      buffer = await ui.ImmutableBuffer.fromUint8List(Uint8List.fromList(data));
+    } else {
+      buffer = await ui.ImmutableBuffer.fromAsset("packages/photo_gallery/images/grey.bmp");
+    }
+    return decode(buffer);
   }
 
   @override

lib/src/models/album.dart

@@ -46,7 +46,7 @@ class Album {
   /// Get thumbnail data for this album.
   ///
   /// It will display the lastly taken medium thumbnail.
-  Future<List<int>> getThumbnail({
+  Future<List<int>?> getThumbnail({
     int? width,
     int? height,
     bool? highQuality = false,

test/photo_gallery_test.dart

@@ -70,7 +70,7 @@ void main() {
 
   test('get album thumbnail', () async {
     String albumId = "__ALL__";
-    List result = await PhotoGallery.getAlbumThumbnail(albumId: albumId);
+    List? result = await PhotoGallery.getAlbumThumbnail(albumId: albumId);
     List expected = Generator.generateMockAlbumThumbnail(albumId: albumId);
     expect(result, expected);
   });