Null Values in submit of form when is included in the form

Question

I have I the following two drop down. I want drop down A on item change to populate Drop down B.This works fine but the problem occurs during submission drop down B is null and I am getting a null validation error. What could I be doing wrong?

 <h:form id="set_up_form">
     <p:messages id="infor" showDetail="false" closable="true"/>
     <p:panelGrid columns="2" id="panel_grid_id">
         <h:outputLabel value="Select Group A"/>
         <p:selectOneMenu styleClass="font" id="group_list"  value="#{myBean.group_A_id}" required="true" requiredMessage="Group Not Selected">  
             <f:selectItem  itemLabel="--SELECT REVENUE GROUP--" itemValue="#{null}"/>
             <f:selectItems value="#{myBean.groupA}"/>   
             <f:ajax render="sub_group_list"/> when I remove this tag and hand code group_A_id in  getGroupB() form is submitted successfully
         </p:selectOneMenu>
         <h:outputLabel value="Select SubGroup"/>
         <p:selectOneMenu styleClass="font" id="sub_group_list"  value="#{myBean.group_B_id}"  >  
             <f:selectItem  itemLabel="--SELECT Group B--" itemValue="#{null}"/>
             <f:selectItems value="#{myBean.groupB}"/>                                                                       
         </p:selectOneMenu>
         <p:commandButton   update="set_up_form" icon="ui-icon-plus" id="btnregister" value="Configure Revenue" action="#{mybean.savevalues}"/>
     </p:panelGrid>
 </h:form>

Here is my Managed Bean

public class MyBean {

    private long group_A_id;
    private long group_B_id;

    // getter and setter     
    public Map<String, String> getGroupA() {
        Map<String, String> map = new LinkedHashMap();
        List<GroupA> grp = DBDao.getGroupARecords();// all values need to be populated
        for (GroupA g : grp) {
            String groupdId = "" + g.getId();
            String groupName = g.getName().toUpperCase();
            map.put(groupName, groupdId);

        }
        return map;
    }

    public Map<String, String> getGroupB() {// values depends on group_id selected from group A dropdown
        Map<String, String> map = new LinkedHashMap();
        List<GroupB> grp = DBDao.getGroupARecords("select from TableB where group_A_id='" + group_A_id + "'");// all values need to be populated
        for (GroupB g : grp) {
            String groupdId = "" + g.getId();
            String groupName = g.getName().toUpperCase();
            map.put(groupName, groupdId);

        }
        return map;
    }

    public void saveConfigs() {
        System.out.println("Successfully Processed" + group_A_id + ":" + group_B_id);

    }
}