Please update code
$(document).ready(function () {
$('#<%=imgToCrop.ClientID%>').Jcrop({
onSelect: getAreaToCrop
});
});
Pass the value onselect :getAreaCrop(Passgeting value)
onSelect: getAreaToCrop("Pass geting value here")
Your Error is the don't pass any value and You call function with Argument and here Function getAreaToCrop(c) in c Value null So this kind of error.
function getAreaToCrop(c) {
$('#XCoordinate').val(parseInt(c.x));
$('#YCoordinate').val(parseInt(c.y));
$('#Width').val(parseInt(c.w));
$('#Height').val(parseInt(c.h));
}