Try enclose the variable '' characters.
It shows to Qlik Sense, that it contains a characters.
By the way I think this document will help you:
Got it! I've added the '' as you said and had to add an = to the GetFieldSelections formula. I installed the extension you linked to, but did not end up using it.
The only problem now is if there are multiple selections in the Demographics field this will not work. Therefore, I am using the bi-listbox to limit the selections in the field to 1. Qlik Sense Listbox