Issue
If the list collector field shows an empty box, this means any empty character is passed in the field via client script or UI policies via setValue().
Cause
As shown in the following screenshot, when an empty box shows in the list collector field it means your client script is using a space character to set the value.
Resolution
Identify the client scripts used to set the value for the list collector variable
This example shows the extra space in the g_form.setValue()
g_form.setValue('affected_varibale_name',' ');
To fix the issue, remove the empty space used in the setValue()
g_form.setValue('affected_varibale_name','');