Right now project is using MainWindow.xib in App delegate class as main nib file. It has this code in main.m file
int main(int argc, char *argv[]) { NSAutorelea
In case others find this as I have, the code you actually want to replace nil with is NSStringFromClass([AppDelegate class]) where you would replace AppDelegate with your classname if you have named yours differently.
nil
NSStringFromClass([AppDelegate class])
AppDelegate
Also, don't forget to #import "AppDelegate.h"
#import "AppDelegate.h"