SUMMARY
From a different machine where I have Thunderbird installed I sent myself an encrypted email with a public key attached to it. On the receiving computer I decrypted the message fine but the public key doesn't show up as an attachment. Insteda I see an "Import key" link in the message viewer and clicking it just generates this error on stdout: org.kde.pim.kmail: Can't handle URL: QUrl("x-kmail:/bodypart/2/1.2/pgpkey?action=import")

STEPS TO REPRODUCE
1. setup thunderbird with enigmail and generate a new pgp key
2. in thunderbird compose a new message and make sure to select the option to attach a public key to it: in the popup dialog select the new pgp entry you just created
3. select the option to encrypt and sign the email and send it
4. receive the email on kmail
5. decrypt it as normal
6. in the message viewer there is no attachment shown
7. in the message viewer click the "Import key" link

OBSERVED RESULT
Nothing happens when clicking the link. No attachment is not shown, so the public key is effectively unreachable.

EXPECTED RESULT
When clicking the "Import key" link the attached public key should get imported to the keyring.
Optionally it should be possible to save the public key to the disk so one could import it with kleopatra or gpg.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Gentoo kernel 5.4.38
(available in About System)
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 
Qt Version: 5.14.2

ADDITIONAL INFORMATION