Fix focus navigation exception when the editor is collapsed

Author: miroiu

Nodify/Editor/NodifyEditor.KeyboardNavigation.cs

@@ -91,7 +91,7 @@ private bool TryFindContainerToFocus(ItemContainer? currentElement, TraversalReq
             {
                 var viewport = new Rect(ViewportLocation, ViewportSize);
                 var containers = ItemContainers;
-                containerToFocus = containers.FirstOrDefault(container => container.IsSelectableInArea(viewport, isContained: false))
+                containerToFocus = containers.FirstOrDefault(container => container?.IsSelectableInArea(viewport, isContained: false) is true)
                     ?? containers.First();
             }