If your database gets into an invalid state, you can clear all of Marvin's local data and download a fresh copy of your data from the sync server.  Note: you should never follow these steps if you have cloud sync disabled!

First, right click anywhere on the page and click Inspect:

This opens a sidebar on the right side of your screen with the "Web Inspector".

Click the Application tab at the top of the Web Inspector (you may have to click the » to see more tabs):

Go to the "Clear Storage" tab on the left and you will see how much data Marvin is storing on your computer.

Click the "Clear site data" button at the bottom to clear all Marvin data:

Finally refresh the page, login, and wait for your data to sync.