Nice catch. Although it solves the immediate problem, to me it feels like it doesn't address the underlying issue. The old if (ac.datatype == ANIMCONT_GPENCIL) basically asks the question "can this datatype be showing grease pencil data?". It would be good to apply the "single responsibility" principle here, and move the responsibility of answering this question to its own function. I'm thinking something like bool BKE_animdata_can_have_greasepencil(eAnimCont_Types type). That way, if GP data is ever shown in other anim contexts as well, there's just one place to adjust this.

Do you think you can add that to this patch? It might be cleaner to first introduce

bool BKE_animdata_can_have_greasepencil(eAnimCont_Types type) {
  return ac.datatype == ANIMCONT_GPENCIL;
}

as a non-functional change in one patch, and then change the condition in that function in another patch. That's more work though, so I'll leave the call to you.

Ok, I can do that !

The new function is now written : D16168