Fix crash when returning to the previous screen.

Crash When you return to the previous screen when you are viewing controlled by UIWebViewDelegate.

Example:

```objc
- (void)webViewDidStartLoad:(UIWebView *)webView
{
    [self showNavigationBar:NO];
    [self showToolbar:NO];
}
```

`self.navigationController` is null sometimes. I added a null judgment for that.