我的经验：

- (void)dealloc{
    /*
     Important
     Before releasing an instance of UIWebView for which you have set a delegate,you must first set the UIWebView delegate property to nil before disposing of the UIWebView instance. 
     This can be done,for example,in the dealloc method where you dispose of the UIWebView.
     */
    if (_webView.loading) {
        [self.webView stopLoading];
    }
    _webView.delegate = nil;
}

if the ViewController is a child of a another ViewController,you can
trigger the removal of the ViewController‘s view from the parent
ViewController‘s view with an animation. At the same time,you can
remove the ViewController from its parent and nil out its reference.
at this point ViewController will be nil and viewWillDisappear will
never be called,meaning **the WebView delegate will never be cleaned up
**
Use dealloc and ensure that your WebView is always cleaned up.

其他优秀的链接
ios:EXC_BAD_ACCESS for Webview delegate