Dispose of PopupRoot when detached from tree.

src/Perspex.Controls/Primitives/Popup.cs

@@ -231,6 +231,8 @@ namespace Perspex.Controls.Primitives
         protected override void OnDetachedFromLogicalTree(LogicalTreeAttachmentEventArgs e)
         {
             _topLevel = null;
+            _popupRoot?.Dispose();
+            _popupRoot = null;
         }
 
         /// <summary>

src/Perspex.Controls/Primitives/PopupRoot.cs

@@ -15,7 +15,7 @@ namespace Perspex.Controls.Primitives
     /// <summary>
     /// The root window of a <see cref="Popup"/>.
     /// </summary>
-    public class PopupRoot : TopLevel, IInteractive, IHostedVisualTreeRoot
+    public class PopupRoot : TopLevel, IInteractive, IHostedVisualTreeRoot, IDisposable
     {
         private IDisposable _presenterSubscription;
 
@@ -64,6 +64,12 @@ namespace Perspex.Controls.Primitives
         /// </summary>
         IVisual IHostedVisualTreeRoot.Host => Parent;
 
+        /// <inheritdoc/>
+        public void Dispose()
+        {
+            this.PlatformImpl.Dispose();
+        }
+
         /// <summary>
         /// Hides the popup.
         /// </summary>