Cannot remove an observer for the key path from because it is not registered as an observer

参考链接

问题原因分析：Tabbar一个viewController只是被Init，但从未显示过，所以viewDidLoad并未被执行。但dealloc事件中却调用了removeObserver方法，导致错误。
不过这只是其中一个原因，不覆盖所有情况。

Apple Docs say there is a way to add observer when view is visible only. According to Figure 1 - Valid State Transitions you can use pair viewWillAppear/viewWillDisppear for adding and removing observers. At the same time you can use init/dealloc pair, but not viewDidLoad/dealloc - view can be not loaded, but controller deallocated.