iOS WKWebview的iOS 11以下崩溃问题

An instance of class WKWebView was deallocated while key value observers were still registered with it.

- (void)dealloc{
    //防止iOS11以下奔溃
    [self.locationWebView removeObserver:self.progressView forKeyPath:@"estimatedProgress"];
    [self.locationWebView setNavigationDelegate:nil];
    [self.locationWebView setUIDelegate:nil];
}