Query about Dialog creation

Folks

When I first run my dialog I get:

No entry 'description.ext/MyDialog/controls/Dropdown.arrowEmpty'

But it executes perfectly from then on! I never see that error again. I guess its something to do with inheritance maybe? But I thought I covered that with:

class Dropdown : RscCombo {};

I can live with the error but would be good to understand why. I have everything in description.ext so I could move it over to a .hpp?

Thanks!