title: Crash when checking if NSTimer isValid?
body: <p>I am getting this exception in the console:</p>
  <blockquote>
  <p>Error:</p>
  <p>2015-06-25 23:12:01.841 Copyfeed for Mac[9512:584232]
  -[_NSViewLayoutAux invalidate]: unrecognized selector sent to instance 0x6000001657c0</p>
  </blockquote>
  <p>when checking if my timers are valid/and when invalidating them.</p>
  <pre><code>if ([_staticTimer isValid]) {
      [_staticTimer invalidate];
      _selectionTimer = 
              [NSTimer scheduledTimerWithTimeInterval:2 target:self 
              selector:@selector(hideHUD) userInfo:nil repeats:NO];
  }
  
  if ([_selectionTimer isValid]) {
      [_selectionTimer invalidate];
      _selectionTimer = 
             [NSTimer scheduledTimerWithTimeInterval:2 target:self 
             selector:@selector(hideHUD) userInfo:nil repeats:NO];
  }
  </code></pre>
  <p>This is my new code:</p>
  <pre><code> if (_selectionTimer != nil) {
          [_selectionTimer invalidate];
          _selectionTimer = nil;
          _selectionTimer = [NSTimer scheduledTimerWithTimeInterval:2 target:self selector:@selector(hideHUD) userInfo:nil repeats:NO];
      }
      
      if (_staticTimer != nil) {
          [_staticTimer invalidate];
          _staticTimer = nil;
          _selectionTimer = [NSTimer scheduledTimerWithTimeInterval:2 target:self selector:@selector(hideHUD) userInfo:nil repeats:NO];
      }
  
  
  @property  (strong )NSTimer *staticTimer;
      @property  (strong )NSTimer *selectionTimer;
  </code></pre>
  <p>Now getting this error when I debug with zombie objects on.</p>
  <blockquote>
  <p>2015-06-26 00:39:45.523 Copyfeed for Mac[11191:824502] ***
  -[CFRunLoopTimer release]: message sent to deallocated instance 0x608000175e40</p>
  </blockquote>