fix(docking): improve structural integrity and add null validation

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

@@ -9,6 +9,7 @@ namespace Ghost.Editor.View.Controls.Docking;
 public abstract class DockContainer : DockModule
 {
     private readonly ObservableCollection<DockModule> _children = new();
+    protected bool _isCleaningUp;
     /// <summary>
     /// Gets the collection of child modules.
     /// </summary>
@@ -81,9 +82,42 @@ public abstract class DockContainer : DockModule
         {
             module.Owner = null;
             module.Root = null;
+            if (!_isCleaningUp)
+            {
                 CheckCleanup();
             }
         }
+    }
+
+    public virtual void ReplaceChild(DockModule oldChild, DockModule newChild)
+    {
+        ArgumentNullException.ThrowIfNull(oldChild);
+        ArgumentNullException.ThrowIfNull(newChild);
+        int index = _children.IndexOf(oldChild);
+        if (index < 0) throw new ArgumentException("oldChild not found");
+
+        // Detach newChild from its current owner if any
+        newChild.Owner?.RemoveChild(newChild);
+
+        // Remove oldChild without triggering cleanup
+        _isCleaningUp = true;
+        try
+        {
+            _children.RemoveAt(index);
+            oldChild.Owner = null;
+            oldChild.Root = null;
+
+            newChild.Owner = this;
+            newChild.Root = Root;
+            _children.Insert(index, newChild);
+        }
+        finally
+        {
+            _isCleaningUp = false;
+        }
+        OnChildrenUpdated();
+        CheckCleanup();
+    }
 
     protected virtual void CheckCleanup()
     {
@@ -104,6 +138,10 @@ public abstract class DockContainer : DockModule
             child.Root = null;
         }
         _children.Clear();
+        if (!_isCleaningUp)
+        {
+            CheckCleanup();
+        }
     }
 
     protected override void OnRootChanged()

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

@@ -57,10 +57,12 @@ public class DockPanel : DockContainer
 
             if (owner != null)
             {
-                int index = owner.Children.IndexOf(this);
+                owner.ReplaceChild(this, child);
-                owner.RemoveChild(this);
+            }
-                child.Detach();
+            else if (Root != null && Root.RootPanel == this)
-                owner.InsertChild(index, child);
+            {
+                // If this is the root panel, we can't easily replace it if the child is a DockGroup, 
+                // because RootPanel must be a DockPanel. So we just leave it.
             }
         }
     }

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

@@ -72,6 +72,8 @@ public class DockingLayout : Control
     /// <param name="targetGroup">The target group to add the document to. If null, a suitable group will be found or created.</param>
     public void AddDocument(DockDocument document, DockTarget target, DockGroup? targetGroup = null)
     {
+        ArgumentNullException.ThrowIfNull(document);
+
         if (targetGroup != null && targetGroup.Root != this)
         {
             throw new ArgumentException("targetGroup does not belong to this DockingLayout");