iOS11 [[UIApplication sharedApplication].windows lastObject]

// iOS11后 [[UIApplication sharedApplication].windows lastObject] 无法获取最外层window,导致一些提示层无法显示；尝试之后发现可通过 [[UIApplication sharedApplication].windows firstObject] 来获取最外层window；

// 修改如下：
    
    UIWindow *mainWindow = nil;
    if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 11) {
        mainWindow = [[UIApplication sharedApplication].windows firstObject];
    } else {
        mainWindow = [[UIApplication sharedApplication].windows lastObject];
    }