title: Rg.Plugins.Popup Crashes on closing popup only in UWP
body: <p>I'm using Rg.Plugins.Popup for a while but I can't get it working anymore on UWP for new projects.</p>
  <p>While its working perfectly in Android, I get this error in UWP :</p>
  <pre><code>Exception thrown: 'System.NullReferenceException' in Rg.Plugins.Popup.UWP.dll
  Object reference not set to an instance of an object.
  
  Rg.Plugins.Popup.UWP.dll!Rg.Plugins.Popup.Windows.Renderers.PopupPageRenderer.UpdateElementSize() Line 124
  </code></pre>
  <p>I made a small test repo <a href="https://github.com/BelightWavre/TestRgPlugIN" rel="nofollow noreferrer">here</a>.
  I use Rg.plugins.popup 1.1.5.188, Xamarin 5.0.0.2012 and the targeted plaform in UWP is 10.0.19041.</p>
  <p>Can anyone help me and/or check if you get the same behavior ?</p>
  <p>I also notice that with Rg.plugins.popup version 2.0.0.12, I get another error :
  Error   CS0234  The type or namespace name 'Popup' does not exist in the namespace 'Rg.Plugins.Popup' (are you missing an assembly reference?)</p>
  <p>Here is my initialisation code in UWP, just as describe in the WIKI:
  <a href="https://i.stack.imgur.com/8Dxw1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8Dxw1.png" alt="enter image description here" /></a></p>