Summary:
This fixes a bug where deleting a page with the page id
missing from the URL would cause JS errors to be thrown.
Test Plan:
Verified manually in dev environment. Browser test added
that should hopefully replicate the repro steps and catch
any regressions.
Reviewers: dsagal, paulfitz
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2856