Adding view count for Forum threads?
Design and Skin Customization
what that means is there was no number that came in. The error is saying
"In order to get you what you are after I need to know the _item_id, but it didn't come in."
So the request is to check that you have the right variable. You need to check that the variable that you are expecting to pass in the _item_id actually contains a number.
So in this case {debug} is what you want. You'd put {debug} in the template just above where your {jrCore_counter.....} is to find out whether
$items[0]._item_id contains a number or not.
In this case we know it doesnt because the counter is complaining. So we need to see if the forum's _item_id is there on some other variable.
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question: what file are you editing?