Fix for editor dock crash.

Author: Natanel-Shitrit

UI/MainWindow.xaml.cs

@@ -230,6 +230,7 @@ private void DockingPaneGroup_ChildrenTreeChanged(object sender, ChildrenTreeCha
             // Solution: Set the active LayoutDocumentPane to the first LayoutDocumentPaneGroup avilable child.
             if (e.Change == ChildrenTreeChange.DirectChildrenChanged
                 && !DockingPaneGroup.Children.Contains(DockingPane)
+                && DockingPaneGroup.Children.Count > 0
                 && DockingPaneGroup.Children[0] is LayoutDocumentPane pane)
             {
                 DockingPane = pane;