fix: 开启后台播放后二次播放，未判空导致播放异常

lib/plugin/pl_player/widgets/bottom_control.dart

@@ -72,9 +72,11 @@ class BottomControl extends StatelessWidget implements PreferredSizeWidget {
           ),
           Row(
             children: [
-              PlayOrPauseButton(
-                controller: _,
-              ),
+              controller != null
+                  ? PlayOrPauseButton(
+                      controller: _,
+                    )
+                  : nil,
               const SizedBox(width: 4),
               // 播放时间
               Obx(() {

lib/services/audio_handler.dart

@@ -149,6 +149,8 @@ class VideoPlayerServiceHandler extends BaseAudioHandler with SeekHandler {
     ));
     if (_item.isNotEmpty) {
       _item.removeLast();
+    }
+    if (_item.isNotEmpty) {
       setMediaItem(_item.last);
     }
     if (_item.isEmpty) {