As mentioned by @fabian-z

I noticed that database locking does not work as expected - it only checks the password (and/or keyfile) but not the yubi key (I can even remove it unnoticed by keepassx).
Also, DatabaseOpenWidget does not enable Challenge Response asynchronously for me; the yubikey has to be plugged in when starting keepassx.

keepassx#52 (comment)

Steps to reproduce:

1. Create kdbx with password + yk
2. Add entry / test data and save
3. Lock database
4. Remove yk
5. Unlock database with just password

Possibly other bugs.

Expected behavior:

• Removal of YubiKey after database locking should make it impossible to unlock the database.
• Async detection of re-insertion of YubiKey won't be supported.
• User will have to re-open the file manually. This may be re-explored in the future, but adds to much complexity now.