ios - Kal calendar in storyboard in combination with Tabbar

Question

I am working with a storyboard and a tabbar controller. Inside the storyboard I added a viewController and made it's class AgendaViewController. this Viewcontroller is embeded inside a navigation controller. Now in the .m of my AgendaViewController I have added this piece of code.

calendar = [[KalViewController alloc] init];
calendar.title = @"";
calendar.delegate = self;
dataSource = [[AgendaDatasource alloc] init];
calendar.dataSource = dataSource;

[self.navigationController pushViewController:calendar animated:YES];

This works. but when I push on the tabbarbutton again. (so I refresh my view) I get a white screen. The white screen is just the view of AgendaViewController

Any help?