f7c24c290e
Refactor from e5a921ad9b did not account for the fact that some
`poll_instance` functions may require valid ID pointers in the checked
node. This is the case e.g. of the node group node, as it also checks
all of the nodes inside the referenced nodegroup.
This commit also fixes a logical mistake in the polling check, that
would systematically prevent pasting of nodes with no `poll_instance`
callback.
Pull Request: https://projects.blender.org/blender/blender/pulls/141729