Try until DOM element present

<script>
            
var existCondition = setInterval(function() {
	if (jQuery('.addtocart_modal.modal-slide .modal-content').length) 
	{
	    console.log("Exists!");
	    clearInterval(existCondition);
	    addAnim();
	}
	else{
	    console.log('waiting');
	}
}, 1500); // check every 1500ms

</script>