Creating the Empty Line Command from Kajak project

Hi this maybe a simple question but I was trying to duplicate the "Empty Line" concept that was created in the example "Kajak" project.

I have copied the Abstract command code to the best of my knowledge additionally I have copied Comment which works well, but when I attempt to get the EmptyLine to work. I get "Error: Abstract Concept instance detected. Use one of the subconcepts instead. Concept: AbstractCommand"

Any help is greatly appreciated.

Thanks,

Chris