You need to search for this function:
//remove selected item from the select element
function removeItem(listBoxID, maxDestinationItems, minItemsLeft, errorMsgControl, errorMinMsgControl)
{
var targetLB = document.getElementById(listBoxID);
if (targetLB.length <= minItemsLeft)
{
...
in file: "MonoX\Admin\controls\scripts\JSHelper.js" and you need to add "minItemsLeft = 0;" to this funtion:
//remove selected item from the select element
function removeItem(listBoxID, maxDestinationItems, minItemsLeft, errorMsgControl, errorMinMsgControl)
{
minItemsLeft = 0;
var targetLB = document.getElementById(listBoxID);
if (targetLB.length <= minItemsLeft)
{
...
Please let me know is this was helpful for you.