As per the information provided in bootstrap multiselect dropdown jquery library,
you should be able to use "onDropdownHidden" as long as you are using Bootstrap version 3 and above. Here is what given in the website.
onDropdownHide
This event handler is triggered when the dropdown are hidden.
Both, the onDropdownShow and the onDropdownHide options are not supported when using Twitter Bootstrap 2.3.
So, you may change onChange event with "onDropdownHide" event.
jQuery('#availablePositions').multiselect({
nonSelectedText: 'Select positions',
includeSelectAllOption: true,
includeSelectAllIfMoreThan: 5,
selectAllText: 'Select all positions',
maxHeight: 300,
buttonWidth: '100%',
onDropdownHide: function(event) {
FillDevicesDropdown(jQuery('#availablePositions').val());
}
});
bootstrap multiselect dropdown jquery library website