The Populate dataset modal retrieves datasets too early

1. Open a random corpus that isn't public.
2. Log out.
3. Use the browser's back button to return to the corpus.
4. A notification appears: An error occurred fetching datasets for this corpus.

This notification is caused by the Populate dataset modal, which tries to fetch datasets immediately before the corpus is available. It should wait until corpora are loaded, and give up if the corpus does not exist after they are.