fix exception on popup dispose.

src/Avalonia.Native/PopupImpl.cs

@@ -9,22 +9,14 @@ namespace Avalonia.Native
 {
     public class PopupImpl : WindowBaseImpl, IPopupImpl
     {
-        IAvnPopup _native;
-
         public PopupImpl(IAvaloniaNativeFactory factory)
         {
             using (var e = new PopupEvents(this))
             {
-                Init(_native = factory.CreatePopup(e), factory.CreateScreens());
+                Init(factory.CreatePopup(e), factory.CreateScreens());
             }
         }
 
-        public override void Dispose()
-        {
-            _native.Dispose();
-            base.Dispose();
-        }
-
         class PopupEvents : WindowBaseEvents, IAvnWindowEvents
         {
             readonly PopupImpl _parent;

src/Avalonia.Native/WindowImplBase.cs

@@ -20,7 +20,6 @@ namespace Avalonia.Native
     {
         IInputRoot _inputRoot;
         IAvnWindowBase _native;
-        bool _isClosed;
         private object _syncRoot = new object();
         private bool _deferredRendering = false;
         private bool _gpu = false;
@@ -254,15 +253,11 @@ namespace Avalonia.Native
 
         public virtual void Dispose()
         {
-            if (!_isClosed)
-            {
-                _isClosed = true;
-                _native?.Close();
-                _native?.Dispose();
-                _native = null;
+            _native?.Close();
+            _native?.Dispose();
+            _native = null;
 
-                (Screen as ScreenImpl)?.Dispose();
-            }
+            (Screen as ScreenImpl)?.Dispose();
         }