fix(docking): improve root cleanup and simplify DockPanel cleanup logic

src/Editor/Ghost.Editor/View/Controls/Docking/DockContainer.cs

@@ -132,9 +132,17 @@ public abstract class DockContainer : DockModule
     /// </summary>
     protected virtual void CheckCleanup()
     {
-        if (_children.Count == 0)
+        if (Children.Count == 0)
         {
-            Owner?.RemoveChildInternal(this, true);
+            if (Owner != null)
+            {
+                Owner.RemoveChildInternal(this, true);
+            }
+            else if (Root != null && Root.RootModule == this)
+            {
+                Root.RootModule = null;
+                Root.NotifyLayoutEmpty();
+            }
         }
     }
 

src/Editor/Ghost.Editor/View/Controls/Docking/DockPanel.cs

@@ -46,15 +46,9 @@ public class DockPanel : DockContainer
 
     protected override void CheckCleanup()
     {
-        if (Children.Count == 0)
-        {
-            if (Root != null && Root.RootModule == this)
-            {
-                Root.NotifyLayoutEmpty();
-            }
         base.CheckCleanup();
-        }
+
-        else if (Children.Count == 1)
+        if (Children.Count == 1)
         {
             var child = Children[0];
             var owner = Owner;