title: scheduledTimer not firing if I navigate to two viewcontrollers
body: <p>I have a weak scheduledTimer:</p>
  
  <pre class="lang-swift prettyprint-override"><code>weak var timer = Timer.scheduledTimer(timerInterval: 2.0, target: self, selector: #selector(fire), userInfo: ["id",id], repeats: false)
  </code></pre>
  
  <p>If I begin this timer and navigate to only one viewController, it will fire in the background with no issue. However, if I navigate to a second viewController before the timer fires, it will never go off. Any idea why this is happening?</p>