title: Video playback fails - [NSURL initFileURLWithPath:]: nil string parameter
body: <p>I'm struggling to understand why this isn't working :/ Everytime I run the project the app crashes throwing me a 'NSInvalidArgumentException', reason: '<em>*</em> -[NSURL initFileURLWithPath:]: nil string parameter'</p>
  
  <p>I've followed a tutorial (I'm pretty new to this) and it worked for him and the code is exactly the same.. Can anyone explain whats going on?</p>
  
  <p>.h file </p>
  
  <pre><code>#import &lt;UIKit/UIKit.h&gt;
  #import &lt;MediaPlayer/MediaPlayer.h&gt;
  #import &lt;QuartzCore/QuartzCore.h&gt;
  
  @interface FirstViewController : UIViewController {   
      MPMoviePlayerViewController *playerController;
  }
  -(IBAction)playVideo;
  @end
  </code></pre>
  
  <p>.m file </p>
  
  <pre><code>#import "FirstViewController.h"
  
  @interface FirstViewController ()
  
  @end
  
  @implementation FirstViewController
  
  {
      MPMoviePlayerController *mpc;
  
  }
  
  
  
  - (IBAction)playButton:(id)sender {
  
      NSString *stringPath = [[NSBundle mainBundle]pathForResource:@"intro" ofType:@"MP4"];
      NSURL *url = [NSURL fileURLWithPath:stringPath];
  
      if(url != nil){
  
      mpc = [[MPMoviePlayerController alloc]initWithContentURL:url];
  
      [mpc setMovieSourceType:MPMovieSourceTypeFile];
  
      [[self view]addSubview:mpc.view];
  
      [mpc setFullscreen:YES];
  
      [mpc play];
  
      }
      else{
          NSLog(@"URL not found");
  
      }
  }
  @end
  </code></pre>