The error show : Can't set “visible of self” to "true"

I set the scipt to a shape:

tell application "OmniGraffle"
tell canvas of front window
	set lstLayers to (layers of it) as list
	repeat with i from 1 to number of items in lstLayers
		set this_item to item i of lstLayers
		-- insert actions here
		set visible of this_item to false
		if name of this_item contains "root" then
			set visible of this_item to true
		end if			
	end repeat		
end tell		

end tell
set visible of self to true