fix old theme with revslider on php7

Hi,
i have this problem on a old theme(not support anymore) with revolutionslider the frontend seems ok (i have update revslider) but on the backend it’s not load the option of revslider the problematic file is page_additional_field.php and code is

			<?php 
			if(class_exists('RevSlider')){
				$slider = new RevSlider();
				$arrSliders = $slider->getArrSlidersShort();
						
				if(empty($arrSliders)){
					_e('No sliders found, Please create a slider', 'dev_reol');
				}
				else{
					$select = UniteFunctionsRev::getHTMLSelect($arrSliders,$revslider_alias,'name="revslider_alias" id="revslider_alias"',true);
?>

wordpress report Fatal error: Uncaught Error: Call to undefined method UniteFunctionsRev::getHTMLSelect() on gutemberg editor is blank page on classic only the field of revslider don’t work, how fix this?