title: MBProgressHUD crash "The view used in the MBProgressHUD initializer is nil"
body: <p>someone uses MBProgressHUD ? I want to use the animation showWithLabelMixed in the example provided with MBProgressHUD librairie. i copied this code from the exemple to my project .</p>
  
  <pre><code>-(IBAction)showActivity:(id)sender
  {
      NSLog(@"Show Activity");
      [self showWithLabelMixed];
  
  }
  
  - (void)showWithLabelMixed{
  
      HUD = [[MBProgressHUD alloc] initWithView:self.navigationController.view];
      [self.navigationController.view addSubview:HUD];
  
      HUD.delegate = self;
      HUD.labelText = @"Conecting";
      HUD.minSize = CGSizeMake(135.f, 135.f);
  
      [HUD showWhileExecuting:@selector(myMixedTask) onTarget:self withObject:nil animated:YES];
  }
  
  
  - (void)myTask {
      // Do something usefull in here instead of sleeping ...
      sleep(3);
  }
  
  - (void)myProgressTask {
      // This just increases the progress indicator in a loop
      float progress = 0.0f;
      while (progress &lt; 1.0f) {
          progress += 0.01f;
          HUD.progress = progress;
          usleep(50000);
      }
  }
  
  
  
  
  - (void)myMixedTask {
      // Indeterminate mode
      sleep(2);
      // Switch to determinate mode
      HUD.mode = MBProgressHUDModeDeterminate;
      HUD.labelText = @"Progress";
      float progress = 0.0f;
      while (progress &lt; 1.0f)
      {
          progress += 0.01f;
          HUD.progress = progress;
          usleep(50000);
      }
      // Back to indeterminate mode
      HUD.mode = MBProgressHUDModeIndeterminate;
      HUD.labelText = @"Cleaning up";
      sleep(2);
      // The sample image is based on the work by www.pixelpressicons.com, http://creativecommons.org/licenses/by/2.5/ca/
      // Make the customViews 37 by 37 pixels for best results (those are the bounds of the build-in progress indicators)
      HUD.customView = [[[UIImageView alloc] initWithImage:[UIImage imageNamed:@"37x-Checkmark.png"]] autorelease];
      HUD.mode = MBProgressHUDModeCustomView;
      HUD.labelText = @"Completed";
      sleep(2);
  }
  </code></pre>
  
  <p>But i have a crash </p>
  
  <blockquote>
    <p>Terminating app due to uncaught exception
    'MBProgressHUDViewIsNillException', reason: 'The view used in the
    MBProgressHUD initializer is nil.'
    can you help me please ?</p>
  </blockquote>