Creating Field Inside Span Tag With Dynamic ID

I have a php script that lists some items which can be checked or unchecked. Now, I am trying to make a javascript that writes a comment field in a certain "span" - comm_'.$i.', where $i starts from 0 - if the check box is already checked (info retrieved from the database) or if the user checks the check box (when the user unchecks the check box the comment field should disappear). The problem is that I am unable to make it work properly (when I check or uncheck a box it doesn't work at all), I mean it only works for the element above, so if box_1 is checked it can only create the comment field in comm_0.

(Part of the) PHP code:
Code:
//Listing the items
<span class="olistr">
<span class="olistc" id="op">
<input type="checkbox" name="box_'.$i.'" " '.($row['state'] == 0?'checked':'').' onClick=javascript:ckState(this,"comm_","Comment",'.$i.')>
</span>
</span> .....

The error I get is - this is similar to the errors I get when checking and unchecking a check box:
Message: 'null' is null or not an object
Line: 64 (this would be line comSpc.innerHTML = 'something1';)
Char: 4
Code: 0