title: AdMob banner disappears after i reopen the app
body: <p>I have a view controller (iPad only) with a banner view implemented using storyboard (I followed the official tutorial).</p>
  
  <p>The ads load fine, the only problem is when I close the app (multitasking): when I reopen it the ad view disappears and doesn't show any ads. </p>
  
  <p>This is happening even with test ads.</p>
  
  <p>My project uses Swift, Xcode 7.2 and AdMob SDK 7.2.0</p>