Select2 Not Showing options

Question

I'm updating two websites both built within YII framework, one has the older version installed and has the new version. I'm facing almost similar problem regarding the Select2 plugin. when using ajax pop-up, on the website with the old version the select options don't show and on the one with a newer version - I can't focus on the search field inside the select box.

I have tried using this code that I found everywhere: $.fn.modal.Constructor.prototype.enforceFocus = function() {};

or

<select id="mySelect2">

</select>

$('#mySelect2').select2({
    dropdownParent: $('#myModal')
});

But they don't work