error checking for activity id

While testing porting over paint
got error that the activity get_id was returning null
added in a check, see also change requests for paint to get running
https://git.sugarlabs.org/~h4ck3rm1k3/paint/h4ck3rm1k3s-paint

see my standalone test instructions here 
https://github.com/h4ck3rm1k3/sugar-toolkit-gtk3/blob/standalone/README