After last update of dhtmlx there is bug with attaching layout to tabbar’s tab.
Demo is there: DHTMLX Snippet Tool
On previous available version 8.0.3 it works well.
(updated a snippet and commect)
https://snippet.dhtmlx.com/t164hq2s
If you are attaching a dhx component to a dhx cell (window/layout/tabbar) you should not define any container for this component in its constructor, but use the attach() method