iOS：9.0 wkWebView.scrollView.delegate over-released

Cannot form weak reference to instance (0x110c5dc0a) of class BViewContrller.
It is possible that this object was over-released,
or is in the process of deallocation.

webView.scrollView.delegate = self;

- (void)dealloc{
    _wkWebView.scrollView.delegate = nil;
    
}