Problem is here
foreach (RepeaterItem items in repeat.Items)
{
HiddenField comment_like = (HiddenField)items.FindControl("comment_id");
Your hidden field id is
comment_like but you are trying to find hidden field with id
comment_id which not there.
Change this to
foreach (RepeaterItem items in repeat.Items)
{
HiddenField comment_like = (HiddenField)items.FindControl("comment_like");
Hope, this helps :)