how to clear variable?

Hi all,

Im using variable (customer) which is influencing my plannning layout as filter. When I run planning application variable is empty and all customers are showing in planning layout. Then I select one customer as filter and it works fine as I expected - but when I need to delete this filter to see all customers again it is not working - system doesn't allow me to have "empty" selection again.

do you know how to overcome this to have same behavior like in standard reporting, where empty condition means all records?