Only allow the player layer observer to be cleared if it's set (#907)

ios/RCTVideo.m

@@ -18,6 +18,7 @@ static NSString *const timedMetadata = @"timedMetadata";
   BOOL _playerItemObserversSet;
   BOOL _playerBufferEmpty;
   AVPlayerLayer *_playerLayer;
+  BOOL _playerLayerObserverSet;
   AVPlayerViewController *_playerViewController;
   NSURL *_videoURL;
 
@@ -794,6 +795,7 @@ static NSString *const timedMetadata = @"timedMetadata";
       // resize mode must be set before layer is added
       [self setResizeMode:_resizeMode];
       [_playerLayer addObserver:self forKeyPath:readyForDisplayKeyPath options:NSKeyValueObservingOptionNew context:nil];
+      _playerLayerObserverSet = YES;
 
       [self.layer addSublayer:_playerLayer];
       self.layer.needsDisplayOnBoundsChange = YES;
@@ -832,7 +834,10 @@ static NSString *const timedMetadata = @"timedMetadata";
 - (void)removePlayerLayer
 {
     [_playerLayer removeFromSuperlayer];
+    if (_playerLayerObserverSet) {
       [_playerLayer removeObserver:self forKeyPath:readyForDisplayKeyPath];
+      _playerLayerObserverSet = NO;
+    }
     _playerLayer = nil;
 }