The issue with cascading dropdown lists using ajax and HDIV - spring-mvc

Question

I　integrated HDIV and Spring MVC. Now I have a form on which there are three dropdown lists contract, taskorder and subtask. The select change of contract will update the content of taskorder dropdown list via ajax and then the select change of the task order will update the content of dropdown list subtask via ajax. The form looks like this:
<c:url var="url" value="/admin/user/newstafftask" >
<c:param name="_MODIFY_HDIV_STATE_" value="${taskOrder.id}" />
</c:url>
<form:form action="${url}" commandName="newRequestStaffTasks">
<form:hidden path="subId" />
<table >
<tr>
<td> <label>Contract </label></td>
<td>
<form:select path="newRequestStaffContract.id">
<option></option>
<form:options items="${contractList}" itemValue="id" itemLabel="contract.contractNbr" />
</form:select>
</td>
</tr>
<tr>
<td> <label>Task Order </label></td>
<td>
<form:select path="taskOrder.id">
</form:select>
</td>
</tr>
<tr>
<td> <label>SubTask Code </label></td>
<td>
<form:select path="subtask.subTaskId">
<option></option>
</form:select>
</td>
</tr>
</table>
</form:form>
When I tried to submit the form, I got the error message "INVALID_PARAMETER_NAME" about the taskOrder.id. I understand the problem is the values of two dropdown lists were updated on the client side.I tried _MODIFY_HDIV_STATE_, but it did not work with this case. So I am not sure how to deal with it now. Please help. Thanks.
Update 1
I already exclude the Ajax request from the HDIV validation. Below is the code snippet that update taskOrder list when the contract select change:
$(".modal-body").on("change", "#newRequestStaffContract\\.id", function() {
getTaskOrderByContract($(this));
});
function getTaskOrderByContract(o) {
contractId = o.val();
if (contractId) {
$.get("../../ajax/getTaskOrderByContract", {
contractId : contractId
}
, function(data) {
var options = '<option value=""></option>';
for (var i = 0; i < data.length; i++) {
options += '<option value="' + data[i].id + '">' +data[i].taskNum+ '</option>';
}
$('#taskOrder\\.id').html(options);
});
};
};