I assume that MainViewController is a subclass of UIViewController so it has to have the view property. Verify that the header file for your view controller is correct.
Reconnect the view outlet in the nib. I assume you have the Class of your File's Owner set to MainViewController.
I came across this yesterday. I had changed the Class of the UIView that was set as File's Owner's view property and got this warning. Simply setting the view outlet again fixed it.