Detach PopupRoot from logical tree...

When parent `Popup` is detached. Together with previous comit also fixes
#706.

src/Avalonia.Controls/Primitives/Popup.cs

@@ -268,8 +268,13 @@ namespace Avalonia.Controls.Primitives
         {
             base.OnDetachedFromLogicalTree(e);
             _topLevel = null;
-            _popupRoot?.Dispose();
+            
-            _popupRoot = null;
+            if (_popupRoot != null)
+            {
+                ((ISetLogicalParent)_popupRoot).SetParent(null);
+                _popupRoot.Dispose();
+                _popupRoot = null;
+            }
         }
 
         /// <summary>

tests/Avalonia.Controls.UnitTests/Primitives/PopupTests.cs

@@ -166,6 +166,24 @@ namespace Avalonia.Controls.UnitTests.Primitives
             }
         }
 
+        [Fact]
+        public void PopupRoot_Should_Be_Detached_From_Logical_Tree_When_Popup_Is_Detached()
+        {
+            using (CreateServices())
+            {
+                var target = new Popup();
+                var root = new TestRoot { Child = target };
+
+                target.Open();
+
+                var popupRoot = (ILogical)target.PopupRoot;
+
+                Assert.True(popupRoot.IsAttachedToLogicalTree);
+                root.Child = null;
+                Assert.False(((ILogical)target).IsAttachedToLogicalTree);
+            }
+        }
+
         [Fact]
         public void PopupRoot_Should_Have_Template_Applied()
         {