rBec3357e03ab1 introduced multi-object snapping.
Seems like this was done without mixed-mode selections in mind.
So code assumed that all selected objects are actually armatures [which
can fail].
In 2.7 this was not a problem, because code only took active object into
account, 2.8 was iterating over all selected_editable_objects.
Now just iterate over objects in posemode instead
Reviewers: brecht, dfelinto
Maniphest Tasks: T61051
Differential Revision: https://developer.blender.org/D4287
b294600ddc