title: AudioKit convolution
body: <p>I'm trying to use the Convolution Reverb in a mac os app.
  AudioKit 4.03</p>
  
  <p>The playground example works for me, but when I try to replicate it in my app, I get this error, and no audio.</p>
  
  <p>2017-11-18 20:21:36.116436-0500 convolutionVerb testing[37554:4533072] [avae] AVAEInternal.h:69:_AVAE_Check: required condition is false: [AVAudioEngine.mm:348:AttachNode: (node != nil)]
  2017-11-18 20:21:36.116642-0500 convolutionVerb testing[37554:4533072] Failed to set (contentViewController) user defined inspected property on (NSWindow): required condition is false: node != nil</p>
  
  <p>Here's my code (which is just a slightly modified version of the AudioKit playground)</p>
  
  <p>class ViewController: NSViewController {</p>
  
  <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
  
  
  
      do {
           file = try AKAudioFile(readFileName: "SAMPLES/Bell.wav")
           player = try AKAudioPlayer(file: file)
      }
      catch {print("DIDN'T LOAD")}
  
      player.looping = true
  
  
  
  
      let stairwell = bundle.url(forResource: "stairwell", withExtension: "wav", subdirectory: "Impulse Responses")
          let dish = bundle.url(forResource: "dish", withExtension: "wav", subdirectory: "Impulse Responses")
  
          stairwellConvolution = AKConvolution(player,impulseResponseFileURL: stairwell!,partitionLength: 8_192)
          dishConvolution = AKConvolution(player,impulseResponseFileURL: dish!,partitionLength: 8_192)
  
  
          mixer = AKDryWetMixer(stairwellConvolution, dishConvolution, balance: 0.5)
      dryWetMixer = AKDryWetMixer(player, mixer, balance: 0.5)
  
  
  
      AudioKit.output = dryWetMixer
      AudioKit.start()
  
  
      stairwellConvolution.start()
      dishConvolution.start()
  
      player.play()
  
  
      // Do any additional setup after loading the view.
  }
  </code></pre>