UISeachDisplayController在UISearchBar下显示白框？

Question

当我点击UISearchBar时，它下面会出现一个白框，但在随后调用UISearchBarDelegate方法时，白框会消失。它仅在用户首次单击UISearchBar时显示。以下是我初始化UISearchDisplayController和UISearchBar的代码。如何删除此白框？

- (void)viewDidLoad {

    [super viewDidLoad];

    self.searchBar = [[UISearchBar alloc]initWithFrame:CGRectMake(0, 0, 320, 44)];
    self.searchBar.delegate = self;
    _isSearching = NO;
    self.searchCntrl = [[UISearchDisplayController alloc]initWithSearchBar:self.searchBar contentsController:self];
    self.searchCntrl.delegate = self;
    self.searchCntrl.searchResultsDataSource = self;
    self.searchCntrl.searchResultsDelegate = self;
    self.searchCntrl.displaysSearchBarInNavigationBar = YES;


}