WebView mainFrame方法返回nil

作为singleton特殊web浏览器的一部分,我在窗口控制器中有以下代码

+ (id)sharedPurchaseController {
    static SomeController *sharedController = nil;

    if (!sharedController) {
        sharedController = [[SomeController alloc] initWithWindowNibName:@"anXIB"];
    }

    return sharedController;
}

- (void)awakeFromNib{
    shouldReloadWeb = NO;
    [progressIndicator startAnimation:nil];
    NSString* urlText = [NSString stringWithString:@"http://www.google.com"];
    [[webView mainFrame] loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:urlText]]];
}

-(void)windowWillClose:(NSNotification *)notification{
    shouldReloadWeb = YES;
}

- (void)windowDidBecomeKey:(NSNotification *)notification{
        if (shouldReloadWeb){
        WebFrame* aFrame = [webView mainFrame]; //<-- Returns nil second time
        [[webView mainFrame] reload];
        shouldReloadWeb = NO;
    }
}

[webView mainFrame]

如果我注释掉创建单例的代码,一切都会按预期工作。

有没有办法创建一个带有应用程序的单例浏览器,同时将WebView保留在Nib中?