Caused by rB64583f3e8d0f: Animation: set Action `idroot` at assignment instead of just at evaluation.
It is apparently valid to call 'what_does_obaction' without an action.
While this sounds like it would not make too much sense, that function
will actually set up the workob in other ways than just evaluating the
action on it, so we cannot skip it entirely. It might make sense to
split this up in two functions in the future ('workob_setup' and
'workob_action_evaluate' or similar) and then skip the later if we have
no action to evaluate.
For now, just skip the call to 'BKE_animdata_action_ensure_idroot'
introduced in rB64583f3e8d0f here, to avoid the crash.