I've been banging my head against this all day, it seems like something simple but I can't figure it out.

I've got an iOS app that I created using the "View-based Application" template in XCode. Here is essentially the code I have:

AppDelegate.h:

#import <UIKit/UIKit.h>
@interface AppDelegate : NSObject <UIApplicationDelegate> {
    UIWindow *window;
    MainViewController *viewController;
}
@property (nonatomic, retain) IBOutlet UIWindow *window;
@property (nonatomic, retain) IBOutlet MainViewController *viewController;
@end

AppDelegate.m:

#import "AppDelegate.h"
#import "MainViewController.h"
@implementation AppDelegate
@synthesize window, viewController;
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
    [self.window addSubview:viewController.view];
    [self.window makeKeyAndVisible];
    return YES;
}
- (void)dealloc {
    [viewController release];
    [window release];
    [super dealloc];
}
@end

MainViewController.h:

#import <UIKit/UIKit.h>
@interface MainViewController : UIViewController {
}
-(IBAction) button:(id)sender;
@end

MainViewController.m:

#import "MainViewController.h"
#import "ModalViewController.h"
@implementation MainViewController
...
-(IBAction) button:(id)sender {
    ModalViewController *mvc = [[[ModalViewController alloc] initWithNibName:NSStringFromClass([ModalViewController class]) bundle:nil] autorelease];
    [self presentModalViewController:mvc animated:YES];
}
@end

There's nothing of interest in the ModalViewController.

So the modal view should display when the button is pressed. When I press the button, it hangs for a second then crashes back to the home screen with no error message.

I am stumped, please show me what I'm doing wrong!