When the POSELIB_OT_create_pose_asset operator was displayed in the
UI, for instance pressing F3 after deleting an object, the poll method
tried to access the mode of a None object, leading to the following
error message being displayed:
```
Traceback (most recent call last):
File ".../3.5/scripts/addons/pose_library/operators.py", line 63, in poll
if context.object.mode != "POSE":
AttributeError: 'NoneType' object has no attribute 'mode'
```
This commit adds a check for whether an object is currently active.