thumbnail: Fix runtime warning getting a preview icon
If we tried to get a preview thumbnail for a file, but that preview icon did not exist, then we'd generate a warning trying to add a reference to that GIcon.
Fixes: e629e46a
If we tried to get a preview thumbnail for a file, but that preview icon did not exist, then we'd generate a warning trying to add a reference to that GIcon.
Fixes: e629e46a