Cannot remove an observer产生的崩溃

[yuhengZhu]

Cannot remove an observer <DFPlayer 0x2811e1700> for the key path "status" from <AVPlayerItem 0x282ff5220> because it is not registered as an observer.
(null)

_playerItem removeObserver的时候发生的崩溃，这种崩溃怎么来查呢。涉及到的具体代码：

• (void)setPlayerItem:(AVPlayerItem *)playerItem{
  if (_playerItem == playerItem) {
  return;
  }
  if (_playerItem) {
  [[NSNotificationCenter defaultCenter] removeObserver:self name:AVPlayerItemDidPlayToEndTimeNotification object:nil];
  [_playerItem removeObserver:self forKeyPath:DFStatusKey];
  [_playerItem removeObserver:self forKeyPath:DFLoadedTimeRangesKey];
  [_playerItem removeObserver:self forKeyPath:DFPlaybackBufferEmptyKey];
  [_playerItem removeObserver:self forKeyPath:DFPlaybackLikelyToKeepUpKey];
  }
  _playerItem = playerItem;
  if (playerItem) {
  [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(df_playerDidPlayToEndTime:) name:AVPlayerItemDidPlayToEndTimeNotification object:nil];
  [playerItem addObserver:self forKeyPath:DFStatusKey options:NSKeyValueObservingOptionNew context:nil];
  [playerItem addObserver:self forKeyPath:DFLoadedTimeRangesKey options:NSKeyValueObservingOptionNew context:nil];
  [playerItem addObserver:self forKeyPath:DFPlaybackBufferEmptyKey options:NSKeyValueObservingOptionNew context:nil];
  [playerItem addObserver:self forKeyPath:DFPlaybackLikelyToKeepUpKey options:NSKeyValueObservingOptionNew context:nil];
  }
  }