It was unclear when the bone eyedropper could be used
since it stayed active even though it would be impossible to pick something.
The limitation is that an armature has to be in pose or edit mode to pick from.
Since that is the case we can check in the poll function if the active
object is an armature and if that armature is either in pose or edit mode.
Follow up to: #138182
Pull Request: https://projects.blender.org/blender/blender/pulls/138498
6a31554394