Don't allow using a closed window as a parent/owner.

src/Avalonia.Controls/Window.cs

@@ -635,6 +635,11 @@ namespace Avalonia.Controls
                 throw new InvalidOperationException("Cannot re-show a closed window.");
             }
 
+            if (parent != null && parent.PlatformImpl == null)
+            {
+                throw new InvalidOperationException("Cannot Show a Window with a closed parent.");
+            }
+
             if (IsVisible)
             {
                 return;
@@ -709,6 +714,11 @@ namespace Avalonia.Controls
                 throw new ArgumentNullException(nameof(owner));
             }
 
+            if (owner.PlatformImpl == null)
+            {
+                throw new InvalidOperationException("Cannot Show a Window with a closed owner.");
+            }
+
             if (IsVisible)
             {
                 throw new InvalidOperationException("The window is already being shown.");

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -387,6 +387,36 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Calling_Show_With_Closed_Parent_Window_Should_Throw()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var parent = new Window();
+                var target = new Window();
+
+                parent.Close();
+
+                var ex = Assert.Throws<InvalidOperationException>(() => target.Show(parent));
+                Assert.Equal("Cannot Show a Window with a closed parent.", ex.Message);
+            }
+        }
+
+        [Fact]
+        public async Task Calling_ShowDialog_With_Closed_Parent_Window_Should_Throw()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var parent = new Window();
+                var target = new Window();
+
+                parent.Close();
+
+                var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => target.ShowDialog(parent));
+                Assert.Equal("Cannot Show a Window with a closed owner.", ex.Message);
+            }
+        }
+
         [Fact]
         public void Window_Should_Be_Centered_When_WindowStartupLocation_Is_CenterScreen()
         {