update getting album thumbnail and album thumbnail provider

lib/photo_gallery.dart

@@ -84,7 +84,7 @@ class PhotoGallery {
   }
 
   /// Get album thumbnail by album id
-  static Future<List<int>?> getAlbumThumbnail({
+  static Future<List<int>> getAlbumThumbnail({
     required String albumId,
     MediumType? mediumType,
     bool newest = true,
@@ -100,7 +100,8 @@ class PhotoGallery {
       'height': height,
       'highQuality': highQuality,
     });
-    return bytes != null ? new List<int>.from(bytes) : null;
+    if (bytes == null) throw "Failed to fetch thumbnail of album $albumId";
+    return new List<int>.from(bytes);
   }
 
   /// get medium file by medium id

lib/src/image_providers/album_thumbnail_provider.dart

@@ -27,18 +27,18 @@ class AlbumThumbnailProvider extends ImageProvider<AlbumThumbnailProvider> {
 
   Future<ui.Codec> _loadAsync(AlbumThumbnailProvider key, ImageDecoderCallback decode) async {
     assert(key == this);
-    final data = await PhotoGallery.getAlbumThumbnail(
-      albumId: album.id,
-      mediumType: album.mediumType,
-      newest: album.newest,
-      height: height,
-      width: width,
-      highQuality: highQuality,
-    );
-    ui.ImmutableBuffer buffer;
-    if (data != null) {
+    late ui.ImmutableBuffer buffer;
+    try {
+      final data = await PhotoGallery.getAlbumThumbnail(
+        albumId: album.id,
+        mediumType: album.mediumType,
+        newest: album.newest,
+        height: height,
+        width: width,
+        highQuality: highQuality,
+      );
       buffer = await ui.ImmutableBuffer.fromUint8List(Uint8List.fromList(data));
-    } else {
+    } catch (e) {
       buffer = await ui.ImmutableBuffer.fromAsset("packages/photo_gallery/images/grey.bmp");
     }
     return decode(buffer);

lib/src/models/album.dart

@@ -49,7 +49,7 @@ 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

@@ -74,7 +74,7 @@ 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);
   });