后退按钮在iOS 7中不起作用

我在所有ViewController中使用此代码来创建后退按钮：

self.btnBack = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 60, 44)];
[self.btnBack setBackgroundImage:[UIImage imageNamed:@"back.png"] forState:UIControlStateNormal];
[self.btnBack addTarget:self action:@selector(cancel:) forControlEvents:UIControlEventTouchUpInside];

-(void)cancel:(id)sender 
{
    [self.navigationController popViewControllerAnimated:YES];   
}

但是，当我使用控制器进入控制器时，UIWebView需要两次轻击两次才能在ios 6中返回。在ios 7中，当我轻击1次时，UIWebView消失的情况会在导航和第二次点击应用程序崩溃时显示黑屏。

在所有屏幕上，该功能都很好用，也许UIWebView我很特别。请帮忙！

我推webview：

-(IBAction)doPrivacy:(id)sender 
{
    UIStoryboard *storyBoard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
    WebPageViewController *web = (WebPageViewController*)[storyBoard instantiateViewControllerWithIdentifier:@"WebPage"];

    web.hidesBottomBarWhenPushed = YES;
    web.urlToOpen = @"http://dfdfdf.co";

    [self.navigationController pushViewController:web animated:YES];
}