title: Got "valueForUndefinedKey: this class is not key value coding-compliant for the key error" when use NSArrayController
body: <p>I'm working for NSCollectionView by using Swift. When I use a NSArrayController object as content of NSCollectionView. I got follow error:</p>
  
  <blockquote>
    <p>An uncaught exception was raised</p>
    
    <p>[ valueForUndefinedKey:]: this class is not key value coding-compliant for the key plugName .</p>
  </blockquote>
  
  <p>I have no idea why. I create a model named NHPlug and a NSArrayController as follow, here's my code. :</p>
  
  <pre><code>/* Class NHAppController */
  import Cocoa
  
  class NHAppController: NSObject {
      @IBOutlet weak var arrayController: NSArrayController?
  
      var plugs: NSMutableArray?
  
      override func awakeFromNib() {
          var t = NHPlug(plugID: 1, plugName: "test")
          plugs = NSMutableArray()
          arrayController?.addObject(t)
      }
  }
  
  /* Class NSPlug */
  import Cocoa
  
  class NHPlug: NSObject {
  
      var plugID : Int
      var plugName : String
  
      init(plugID: Int, plugName: String) {
          self.plugID = plugID
          self.plugName = plugName;
      }
  
  }
  </code></pre>
  
  <p><strong>If anybody could help, very appreciate for that!</strong></p>
  
  <p>And here if crash stack:</p>
  
  <blockquote>
    <p>-01-21 19:47:07.347 Notifyhub[58369:598950] An uncaught exception was raised 2015-01-21 19:47:07.347 Notifyhub[58369:598950]
    [ valueForUndefinedKey:]: this class
    is not key value coding-compliant for the key plugName . 2015-01-21
    19:47:07.348 Notifyhub[58369:598950] (    0   CoreFoundation<br>
    0x00007fff9220b64c __exceptionPreprocess + 172    1   libobjc.A.dylib<br>
    0x00007fff950966de objc_exception_throw + 43  2   CoreFoundation<br>
    0x00007fff9220b1e9 -[NSException raise] + 9   3   Foundation<br>
    0x00007fff8bd6c5ed -[NSObject(NSKeyValueCoding) valueForUndefinedKey:]
    + 226     4   Foundation                          0x00007fff8bc89e68 -[NSObject(NSKeyValueCoding) valueForKey:] + 385     5   Foundation                          0x00007fff8bca28e3 -[NSObject(NSKeyValueCoding) valueForKeyPath:] +
    324   6   Foundation                          0x00007fff8bca28af
    -[NSObject(NSKeyValueCoding) valueForKeyPath:] + 272  7   AppKit                              0x00007fff8975c8bf -[NSBinder
    valueForBinding:resolveMarkersToPlaceholders:] + 164  8   AppKit<br>
    0x00007fff89760700 -[NSValueBinder
    _adjustObject:mode:observedController:observedKeyPath:context:editableState:adjustState:]
    + 302     9   AppKit                              0x00007fff8976053b -[NSValueBinder _observeValueForKeyPath:ofObject:context:] + 276     10  AppKit                              0x00007fff897603ab
    -[NSTextValueBinder _observeValueForKeyPath:ofObject:context:] + 43   11  Foundation                          0x00007fff8bc63c73
    NSKeyValueNotifyObserver + 382    12  Foundation<br>
    0x00007fff8bc62e48 NSKeyValueDidChange + 463  13  Foundation<br>
    0x00007fff8bc679ad -[NSObject(NSKeyValueObserverNotification)
    didChangeValueForKey:] + 118  14  AppKit<br>
    0x00007fff89eb5e83 -[NSCollectionView newItemForRepresentedObject:] +
    87    15  AppKit                              0x00007fff89ebce03
    -[NSCollectionView _getItemsToDisplay] + 1168     16  AppKit                              0x00007fff89eb5595 -[NSCollectionView setContent:] + 217    17  AppKit<br>
    0x00007fff89e8e965 -[NSCollectionViewBinder _updateContent] + 93  18 
    AppKit                              0x00007fff89e8e9f0
    -[NSCollectionViewBinder _observeValueForKeyPath:ofObject:context:] + 84  19  Foundation                          0x00007fff8bc63c73
    NSKeyValueNotifyObserver + 382    20  Foundation<br>
    0x00007fff8bca22a6 -[NSObject(NSKeyValueObservingPrivate)
    _notifyObserversForKeyPath:change:] + 1142    21  AppKit                              0x00007fff897533ba -[NSController _notifyObserversForKeyPath:change:]
    + 206     22  AppKit                              0x00007fff8984f126 -[NSArrayController didChangeValuesForArrangedKeys:objectKeys:indexKeys:] + 54   23  AppKit
    0x00007fff89a1dbd0 -[NSArrayController
    _insertObject:atArrangedObjectIndex:objectHandler:] + 514     24  AppKit                              0x00007fff89a1d82c -[NSArrayController addObject:] + 153    25 
    Notifyhub                           0x000000010000166a
    _TFC9Notifyhub15NHAppController12awakeFromNibfS0_FT_T_ + 362  26  Notifyhub                           0x00000001000016d2
    _TToFC9Notifyhub15NHAppController12awakeFromNibfS0_FT_T_ + 34     27  CoreFoundation                      0x00007fff9211983f -[NSSet
    makeObjectsPerformSelector:] + 223    28  AppKit<br>
    0x00007fff89ba2ffd -[NSIBObjectData
    nibInstantiateWithOwner:options:topLevelObjects:] + 1216  29  AppKit<br>
    0x00007fff89c22122 -[NSNib
    _instantiateNibWithExternalNameTable:options:] + 677  30  AppKit                              0x00007fff89c22236 -[NSNib
    _instantiateWithOwner:options:topLevelObjects:] + 143     31  AppKit                              0x00007fff89d5f36c -[NSStoryboard
    instantiateControllerWithIdentifier:] + 181   32  AppKit<br>
    0x00007fff896afef4 NSApplicationMain + 840    33  Notifyhub<br>
    0x00000001000011c2 top_level_code + 34    34  Notifyhub<br>
    0x00000001000011fa main + 42  35  libdyld.dyli</p>
  </blockquote>