Bug Description

Since the v3.0.39 update I'm seeing this error when saving a sites' homepage in the CP. Changes to content are saved to disk though.

LOG.error: Argument passed to Statamic\Structures\Tree::page() must be of the type string, null given, called in /home/forge/example-redacted-url.com/vendor/statamic/cms/src/Http/Controllers/CP/Collections/EntriesController.php on line 182

How to Reproduce

Update to v3.0.39
Save the homepage in the CP and note the error notification without a save notification.

Extra Detail

NB: Not seeing this on saving other pages (yet?).

Environment

Statamic 3.0.39 Pro
Laravel 7.29.3
PHP 7.2.24-0ubuntu0.18.04.7
Server Forge Provisioned DO droplet

Install method (choose one):
Clone of previous site which originated from Cool Writings starter kit