如何在jquery bootstrap multiselect中禁用更改时的选项组

<select id="mySelect" multiple data-live-search="true">
  <option value="1">选项1</option>
  <option value="2">选项2</option>
  <option value="3">选项3</option>
  <option value="4">选项4</option>
</select>

$(document).ready(function() {
  $('#mySelect').multiselect({
    onChange: function(option, checked, select) {
      var selectedGroup = $(option).parent('optgroup');
      if (selectedGroup.attr('disabled')) {
        $(option).prop('selected', !checked);
      }
    }
  });
});

<select id="mySelect" multiple data-live-search="true">
  <optgroup label="组1" disabled>
    <option value="1">选项1</option>
    <option value="2">选项2</option>
  </optgroup>
  <optgroup label="组2">
    <option value="3">选项3</option>
    <option value="4">选项4</option>
  </optgroup>
</select>