how to avoid error when using the 'cross' in a selectionbox

Hello! 

I got an erorr message saying “Unable to map 'undefined' to an element within the range of set 'xxx' (in widget 'xxx)

Setup as follows: Set A:= {‘Option 1’,Option 2’}. Element parameter eA with Range of the Set A. 

Create a widget selection box with content eA.

Once I select ‘Option 1’ or ‘Option 2’ and I want to empty the selection box again (preferabele using the ‘cross’ but then I got the error message).  And using the delete button does not empty the element parameter eA. 

So how can I empty a selection box? (i know there if another topic but like I said the delete button does not work)

I’m using aimms 4.91