Fix error in Editor

Source/Editor/CustomEditors/Editors/ModelInstanceEntryEditor.cs

@@ -76,7 +76,7 @@ namespace FlaxEditor.CustomEditors.Editors
 
         private void OnSelectedMaterialChanged()
         {
-            if (_isRefreshing)
+            if (_isRefreshing || _modelInstance == null)
                 return;
             _isRefreshing = true;
             var slots = _modelInstance.MaterialSlots;

Source/Editor/GUI/AssetPicker.cs

@@ -75,6 +75,9 @@ namespace FlaxEditor.GUI
         /// </summary>
         protected virtual void OnSelectedItemChanged()
         {
+            if (IsDisposing)
+                return;
+
             // Update tooltip
             string tooltip;
             if (Validator.SelectedItem is AssetItem assetItem)