We are very sorry but we have a bug in material select. We have a fix for you! We will fix it globally in next release. This fix is easy and fast.
You need to find your material-selecjt.js file.
Next, you need to go to 160 line in material select.
Change icon_url to iconUrl !
It should look like that :

if (iconUrl) {

 var classString = '';
 if (classes) {

 classString = ' class="' + classes + '"';
 }

 if (type === 'multiple') {

 options.append($('<li class="' + disabledClass + '"><img alt="" src="' + iconUrl + '"' + classString + '><span class="filtrable"><input class="form-check-input" type="checkbox"' + disabledClass + '/><label></label>' + option.html() + '</span></li>'));
 } else {

 options.append($('<li class="' + disabledClass + optgroupClass + '"><img alt="" src="' + iconUrl + '"' + classString + '><span class="filtrable">' + option.html() + '</span></li>'));
 }</code>

We are very sorry again about that bug.
Best,
Piotr