don't crash on missing decl. fix for #203.
don't crash on missing decl. fix for #203.

switch-education/pxt,playi/pxt,switch-education/pxt,Microsoft/pxt,switch-education/pxt,switch-education/pxt,playi/pxt,switchinnovations/pxt,Microsoft/pxt,playi/pxt,switch-education/pxt,Microsoft/pxt,Microsoft/pxt,Microsoft/pxt,switchinnovations/pxt,playi/pxt,switchinnovations/pxt,switchinnovations/pxt