如果我在包含外部链接的 UIWebView 中调用 loadHTMLString，然后再次尝试 loadHTMLString，它就会崩溃。
HTML 加载如下：
NSURL *url = [[NSBundle mainBundle] URLForResource:@"help" withExtension:@"html"];
NSString *text = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:nil];
[[self webView] loadHTMLString:text baseURL:nil];
help.html 可以包含一个链接，例如：
如果我点击链接，它会加载页面。但是我在导航栏上有一个“主页”按钮，它再次调用上面的代码（loadHTMLString），这会使应用程序崩溃。
查看外部链接后，有什么方法可以“欺骗” UIWebView 再次加载 HTML 字符串？
参考资料： 在 iOS 中加载 HTML 后 UIWebView 不会返回
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** setObjectForKey: key cannot be nil'
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