title: UIFeedback Haptic Engine called more times than was activated
body: <p>I am using the <code>UIFeedback Haptic Engine</code> with <strong>swift 2.3</strong> like:</p>
  
  <pre><code>let generator = UINotificationFeedbackGenerator()
  generator.notificationOccurred(.Warning)
  </code></pre>
  
  <p>and</p>
  
  <pre><code>let generator = UIImpactFeedbackGenerator(style: .Heavy)
  generator.impactOccurred()
  </code></pre>
  
  <p>Today I got a new kind of error like this, and couldn't find the problem. Do you have any idea?</p>
  
  <pre><code>UIFeedbackHapticEngine _deactivate] called more times than the feedback engine was activated
  </code></pre>
  
  <p>Details:</p>
  
  <pre><code>Fatal Exception: NSInternalInconsistencyException
  0  CoreFoundation                 0x1863e41c0 __exceptionPreprocess
  1  libobjc.A.dylib                0x184e1c55c objc_exception_throw
  2  CoreFoundation                 0x1863e4094 +[NSException raise:format:]
  3  Foundation                     0x186e6e82c -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:]
  4  UIKit                          0x18cc43fb8 -[_UIFeedbackEngine _deactivate]
  5  UIKit                          0x18cad781c -[UIFeedbackGenerator __deactivateWithStyle:]
  </code></pre>