message sent to deallocated inst

报错:...message sent to deallocated instance...补充,除了网上查到,我遇到的是监听重复注销导致的.
reason: 'Cannot remove an observer <XXXViewController 0x10ee11ec0> for the key path "canLeave" from <XXXConversation 0x******00> because it is not registered as an observer.
原因:不能删除这个观察者...........因为它不是一个注册的观察者.