[curt.clifton]

Try this:

on onContext(aContext)
	using terms from application "OmniFocus"
		set contextName to name of aContext
		set contextLocation to location of aContext
		my onContexts(contexts of aContext)
	end using terms from
end onContext

The clue is that location wasn't showing up as a property but as a variable name. All objects have a name, so that line worked OK.