暂时把bug先处理掉了，准备尝试实现最小播放目标

lib/widgets/mini_player_bar.dart

@@ -8,7 +8,9 @@ class MiniPlayerBar extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final service = context.watch<AudioService>();
-    final song = service.currentSong!;
+    final song = service.currentSong;
+
+    if (song == null) return const SizedBox.shrink();
 
     return Container(
       height: 64,
@@ -21,7 +23,6 @@ class MiniPlayerBar extends StatelessWidget {
             offset: const Offset(0, -4),
           ),
         ],
-        // 顶部微渐变（轻阴影）
         gradient: LinearGradient(
           begin: Alignment.topCenter,
           end: Alignment.bottomCenter,
@@ -35,7 +36,6 @@ class MiniPlayerBar extends StatelessWidget {
       child: Row(
         children: [
           const SizedBox(width: 12),
-          // 封面占位（56px）
           Container(
             width: 48,
             height: 48,
@@ -47,7 +47,6 @@ class MiniPlayerBar extends StatelessWidget {
                 const Icon(Icons.music_note, color: Colors.white38, size: 24),
           ),
           const SizedBox(width: 12),
-          // 歌曲信息
           Expanded(
             child: Column(
               mainAxisAlignment: MainAxisAlignment.center,
@@ -75,7 +74,6 @@ class MiniPlayerBar extends StatelessWidget {
               ],
             ),
           ),
-          // 控制按钮
           IconButton(
             icon: Icon(
               service.isPlaying ? Icons.pause : Icons.play_arrow,