In Contacts.ts, in the body of the listContacts() function, line 51 has:
(inError: Error, inDocs: IContact[]) => { ...
This gives me an error, because this.db.find seems to expect the inError to be a union of Error and null.
For the error to disappear, I had to change my code to this:
(inError: Error | null, inDocs: IContact[]) => { ...