If any of the selected collection entries in the outliner is a linked
collection, `outliner_collection_from_tree_element` can return null and
it will crash in `BKE_collection_child_find`. This patch handles this
case by skipping those entries and modified the warning message to show
how many selected entries have failed.
----
This is how the message looks like now:
<img width="746" alt="image.png" src="attachments/b91f2a64-7f2d-42d4-8657-7deaf00e90b6">
Pull Request: https://projects.blender.org/blender/blender/pulls/140847
8a3d773b16