The database connection has been closed explicitly, by calling db.close(), or it was opened with option {autoOpen: false} and db.open() was not yet called upon.
Sample using Promise.catch()
doSomeDatabaseWork().then(result => {
// Success
}).catch('DatabaseClosedError', e => {
// Failed with DatabaseClosedError
console.error ("DatabaseClosed error: " + e.message);
}).catch(Error, e => {
// Any other error derived from standard Error
console.error ("Error: " + e.message);
}).catch(e => {
// Other error such as a string was thrown
console.error (e);
});