Select next/previous item from data validation list using Command Button

I have a data validation list in cell "A1", the validation list is sourced from items which are listed under column "AA". And i also have two command buttons to display the next/previous item from the list while clicking the respective buttons (the same should display in cell A1)

The incell dropdown button working very well to select the desired item. But if i use the command buttons to change the item from the list, from the next time the sheet get activated I am getting the following error message

“Run-time error ' -2147417848(80010108) Method 'add' of object 'validation' failed”

i also wanted the first item from the list to be displayed each time the sheet get activated

The file is attached for reference,

Please help