title: Using Swift 3 Stopping a scheduledTimer, Timer continue firing even if timer is nil
body: <p>We call startTimer function to start a timer. When we wanted to stop it we call stopTimerTest function but after we called stopTimer function the timerTestAction keeps firing. To check the timer condition we used print and print in timerActionTest returns nil.</p>
  
  <pre><code>var timerTest: Timer? = nil
  
  func startTimer () {
      timerTest =  Timer.scheduledTimer(
          timeInterval: TimeInterval(0.3),
          target      : self,
          selector    : #selector(ViewController.timerActionTest),
          userInfo    : nil,
          repeats     : true)
  }
  
  func timerActionTest() {
      print(" timer condition \(timerTest)")
  }
  
  func stopTimerTest() {
      timerTest.invalidate()
      timerTest = nil
  }
  </code></pre>