Java Database Connectivity (JDBC)

hi,
i use h:selectOneListbox and populate them also use button, when i click on button action event it give some error.
My code:

<h:selectOneListbox binding="#{DynamicListBean.std_list_lbx}" id="std_list_lbx" immediate="true"
                            onchange="common_timeoutSubmitForm(this.form, 'std_list_lbx');" required="true" size="5"
                            style="height: 312px; left: 168px; top: 192px; position: absolute; width: 216px" value="#{DynamicListBean.selectedCategory1}" valueChangeListener="#{DynamicListBean.std_list_lbx_processValueChange}">
                            <f:selectItems id="std_lstbxSelectItems" value="#{DynamicListBean.items1}"/>
                        </h:selectOneListbox>

javabean code

  public SelectItem[] getItems1() {
        return items1;
    }

    public void setItems1(SelectItem[] items1) {
        this.items1 = items1;
    }

    public String getSelectedCategory1() {

        return selectedCategory_std_lstBx;
    }

    public void setSelectedCategory1(String selectedCategory1) {
        this.selectedCategory_std_lstBx = selectedCategory1;
    }

    public List getDList1() {
        return stdListbxdList;
    }

    public void setDList1(List list) {
        stdListbxdList = list;
    }

    public ArrayList getDynList1() {
        return dynList1;
    }

    public void setDynList1(ArrayList dynList1) {
        this.dynList1 = dynList1;
    }
     public SelectItem[] populateSelectOneMenu1() {
        connection();
        int n1 = 0;
        String a = "";
        if (con != null) {
            try {
                stmt = con.createStatement();
                String query = "select std_name FROM std_detail where std_id=1001";
                rs = stmt.executeQuery(query);
                if (rs != null) {
                    while (rs.next()) {
                        String data3 = rs.getString("std_name");
                        //String data4 = rs.getString("std_id");
                        stdListbxdList.add(data3);
                    }
                    con.close();
                }
            } catch (Exception e) {
                // out.println("Sorry failed to delete values from the database table. " + e.getMessage());
                }
        }
        List selectItems = new ArrayList();

        Iterator it = stdListbxdList.listIterator();
        while (it.hasNext()) {
            String label = (String) it.next();
            selectItems.add(new SelectItem(label));
        }
        return (SelectItem[]) selectItems.toArray(new SelectItem[0]);
    }


    public String phy_btn_action() {
        // TODO: Replace with your code
        populateSelectOneMenu1();
        return null;
    }

error=========

sourceId=form1:std_list_lbx[severity=(ERROR 2), summary=(form1:std_list_lbx: Validation Error: Value is required.), detail=(form1:std_list_lbx: Validation Error: Value is required.)];|WARNING: FacesMessage(s) have been enqueued, but may not have been displayed.
sourceId=form1:std_list_lbx[severity=(ERROR 2), summary=(form1:std_list_lbx: Validation Error: Value is required.), detail=(form1:std_list_lbx: Validation Error: Value is required.)]

i cant find solution for that.