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Problems with calendar attached to a text input

I have a calendar which attached to a text input. I want to keep text input editable and want to popup calendar when a button/icon is clicked and then set the date in text input. I got the following code but, when I click on calendar image, I get javascript error:
this.parent is null
onMonthSelect()()dhtmlxcalendar.js (line 84)
(?)()()edit (line 1176)
[Break on this error] this.onLanguageLoaded(status)};function...ct = function(value) {if (!isNaN(value))

My code:
<div>
<span>
<input id="question_16" type="text" name="web_form[question_16]" value=""/>
<img onclick="openCalendar_16();" src="/images/calendar.gif"/>
<span id="cal_container_16" style="float: left;"/>
<label for="question_16">MM/DD/YYYY</label>
</span>
</div>
<script>
1
2 var cal_16;
3 add_calendar_16 = function() {
4 cal_16 = new dhtmlxCalendarObject('cal_container_16',false,{isYearEditable :true,isMonthEditable :true});
5 cal_16.setDateFormat('%m/%d/%Y');
6 cal_16.setSkin('yahoolike');
7 dhtmlxEvent(document.body,"click",function(){ cal_16.hide(); }); //hide calendar onclick
8 //dhtmlxEvent(cal_16.con,"click",function(e){ (e||event).cancelBubble=true; }); //block click events inside calendar object
9 //dhtmlxEvent(cal_16.entObj,"click",function(e){ (e||event).cancelBubble=true; }); //block click events inside calendar object
10 cal_16.attachEvent ("onClick", function(date){
11 document.getElementById ('question_16').value = cal_16.getFormatedDate('%m/%d/%Y',date)
12 });
13 }
14 function openCalendar_16() {
15 alert(cal_16);
16 cal_16.isVisible() ? cal_16.hide() : cal_16.show();
17 }
18 Event.observe(window, 'load', add_calendar_16);
19
</script>

Thanks for the reply!
Making suggested change won't work either.
cal_16.draw();
$('cal_container_10').hide();// hide the container

now in onClick I call $('cal_container_10').show(); calendar won't show. I also tried calling cal_16.show() but clicking on icon won't show the calendar. I don't get any javascript errors now.

function openCalendar_10() {