Theres not much to go by with t he code you have provided. Only thing i can think of without looking at more code is that perhaps you have not initialized your view controller (the one in your dictionary) correctly, or at all, or the dictionary you are accesing there is not the correct one...
What is the value of [self navigationController] when pushViewController is called. Is it nil?
What is the value of targetViewController when it is passed to pushViewController. Is it nil?
Finally figured it out......
I had the UINavigationController and the UITabBarController on the same level. After moving the Navigation Controller as a child to the tab bar controller, everything worked. You may add as many navigation controllers to a tab bar as you would like. In IB is is a little confusing. You need to drag the NavigationController into the TabBar to as it as a new Tab Bar item.