iOS (iPad) parent view unloaded on memory warning

Here's the situation. I have a MainControlle which opens a modal XController. When memory warning is received (when XController is displayed) MainController is unloaded (viewDidUnload is called) and you can not go back from XController to MainController.

What could be the problem? How to fix that?

UPDATE:

Is it OK if I just comment a super method (so the 开发者_开发知识库OS will be forced to close other programs first before destroying this view) and set variables to nil only in dealloc?

(void)viewDidUnload
{ 
  // [super viewDidUnload]
}

Yes... just comment everything out as I explained and it works.