Hi i have two WebDropDown control in my form. one is state and another one is city. i selected state means corresponding city loaded to city WebDropDown using javascript. i wrote a code but i got error like "Microsoft JScript runtime error: Unable to get value of the property '_callbackManager': object is null or undefined"

My Code is :

function LoadDropDown() {
            var ddlState = $find('ddlState');

            var Tamilnadu = new Array('--Select--', 'Salem', 'Coimbatore', 'Karur');
            var Kerala = new Array('--Select--', 'Kochin', 'Palakad', 'Kollam');

            var ddlCity = $find('ddlCity');

            switch (ddlState.get_currentValue()) {
                case 'Tamilnadu':

                    for (var i = 0; i < Tamilnadu.length; i++) {
                        var item = ddlCity.get_items().createItem();
                        item.set_text(Tamilnadu[i].toString());
                        item.set_value(i);
                        ddlCity.get_items().add(item);
                        ddlCity.set_selectedItemIndex(0)
                    }
                    break;

                case 'Kerala':

                    for (var i = 0; i < Kerala.length; i++) {
                        var item = ddlCity.get_items().createItem();
                        item.set_text(Kerala[i]);
                        item.set_value(i);
                        ddlCity.get_items().add(item);
                    }
                    break;

                default:
                    ddlCity.options.length = 0;
                    break;
            }
        }

Kindly help me someone please.

Thanks.