AVPlayerItem was deallocated while key value observers were still registered with it.

[S-Freedom]

use RACObserve caused a crash:
creash info:

#56 Thread
NSInternalInconsistencyException

An instance 0x178201b40 of class AVPlayerItem was deallocated while key value observers were still registered with it. Current observation info: <NSKeyValueObservationInfo 0x174a3bca0> ( <NSKeyValueObservance 0x1754523c0: Observer: 0x1704342e0, Key path: playbackLikelyToKeepUp, Options: <New: NO, Old: NO, Prior: YES> Context: 0x1758637c0, Property: 0x175e5df70> )

my code:
//缓冲区播放完成
[RACObserve(self.playerItem, playbackBufferEmpty) subscribeNext:^(id _Nullable x) {
@strongify(self);
BOOL empty = [x boolValue];
if (empty) {
//缓冲区为空
[self callBackDelegateStatus:PlayerStatusBuffering];
}
}];