When the active object had no shapekey data, trying to create a new action from the
Shape Keys mode of the DopeSheet would crash. The segfault here was a silly regression
caused by my earlier Action Stashing work.
However, the old (pre-Action Stashing) code here also wasn't that great either.
While it didn't crash, it would still silently create a new action, even if that
could not get assigned/used anywhere. To prevent both of these problems from
happening again, I've added additional null checks, as well as beefing up the poll
callback here to forbid keyframing
b7afbaf8cc