Ensure Initialized gets called on TopLevels.

src/Perspex.Controls/Primitives/PopupRoot.cs

@@ -84,6 +84,7 @@ namespace Perspex.Controls.Primitives
         /// </summary>
         public void Show()
         {
+            EnsureInitialized();
             PlatformImpl.Show();
             LayoutManager.Instance.ExecuteInitialLayoutPass(this);
             IsVisible = true;
@@ -108,6 +109,16 @@ namespace Perspex.Controls.Primitives
             }
         }
 
+        private void EnsureInitialized()
+        {
+            if (!this.IsInitialized)
+            {
+                var init = (ISupportInitialize)this;
+                init.BeginInit();
+                init.EndInit();
+            }
+        }
+
         private void SetTemplatedParentAndApplyChildTemplates(IControl control)
         {
             var templatedParent = Parent.TemplatedParent;

src/Perspex.Controls/Window.cs

@@ -204,6 +204,7 @@ namespace Perspex.Controls
         {
             s_windows.Add(this);
 
+            EnsureInitialized();
             LayoutManager.Instance.ExecuteInitialLayoutPass(this);
 
             using (BeginAutoSizing())
@@ -236,6 +237,7 @@ namespace Perspex.Controls
         {
             s_windows.Add(this);
 
+            EnsureInitialized();
             LayoutManager.Instance.ExecuteInitialLayoutPass(this);
 
             using (BeginAutoSizing())
@@ -311,5 +313,15 @@ namespace Perspex.Controls
 
             base.HandleResized(clientSize);
         }
+
+        private void EnsureInitialized()
+        {
+            if (!this.IsInitialized)
+            {
+                var init = (ISupportInitialize)this;
+                init.BeginInit();
+                init.EndInit();
+            }
+        }
     }
 }