为什么我的navigationBar中的UIBarbuttonItem有时会消失，但仍然可以响应动作？

Question

在我的Controller Initialize方法＆＃39; initWithXXX＆＃39;我为自定义leftBarbuttonItem写了这个，这个Item有时会消失，但它仍然可以响应之前设置的动作。

self.navigationItem.leftBarButtonItem = self.myleftBarItem;-(UIBarButtonItem *)myleftBarItem{

if (!_myleftBarItem) {
    UIButton *backButton = [UIButton buttonWithType:UIButtonTypeSystem];
    backButton.frame = CGRectMake(3, 0, 44, 44);
    [backButton setImage:[UIImage imageNamed:@"navi_back"] forState:UIControlStateNormal];
    backButton.imageEdgeInsets = UIEdgeInsetsMake(0, -20, 0, 0);
    [backButton setTitle:HXGetStringWithKeyFromTable(@"STR_BACK",nil) forState:UIControlStateNormal];
    backButton.titleEdgeInsets = UIEdgeInsetsMake(0, -13, 0, 0);
    backButton.titleLabel.font = [UIFont systemFontOfSize:16];
    [backButton addTarget:self action:@selector(goBackToClick:) forControlEvents:UIControlEventTouchUpInside];
    _myleftBarItem = [[UIBarButtonItem alloc] initWithCustomView:backButton];
}
return _leftBarIterm;}