in your "reply" action, you defined @parent:
@parent = Topic.find(params[:id])
but in your replyTopic action, you define "parent", which is a local
variable, and not @parent:
parent = Topic.find(params[:parentID])
@topic = Topic.new(params['topic'])
therefore Rails has no @prent variable in the "reply.rhtml" view to
fill the forms, and throws an error.
the problem should be simply fixed by changing "parent" into "@parent"
in the replyTopic action