Hi,
Run below example.
Example : https://dojo.telerik.com/oJoCOCiR/64
(1) Go to Category Column.
(2) Click on Cell and Open Combobox and select 'Block' option.
(3) Now again navigate to same cell and click on 'UnBlock' option.
(4)Now again navigate to same cell and click but do not select any option and switch to another cell.
It will select by default value 'Undefine'.