fix android crash when back from auth app

Flutter/android/src/main/java/com/tencent/vod/flutter/tools/TXFlutterEngineHolder.java

@@ -93,9 +93,11 @@ public class TXFlutterEngineHolder {
             public void onActivityDestroyed(@NonNull Activity activity) {
                 synchronized (mActivityList) {
                     int index = findIndexByAct(activity);
+                    if (index >= 0) {
                         mActivityList.remove(index);
                     }
                 }
+            }
         };
         ((Application)binding.getApplicationContext()).registerActivityLifecycleCallbacks(mLifeCallback);
     }