SUMMARY
The Unified Mailbox Agent plugin is unresponsive, does not stay active after activating it in the plugins menu, and cannot be modified in that menu.

STEPS TO REPRODUCE
1. Go to Settings --> Configure KMail --> Plugins tab
2. Check Unified Mailbox Agent on --> click Apply
3. Click on the Settings button for the Unified Mailbox Agent

OBSERVED RESULT
The settings window for the Unified Mailbox Agent does not open, and the plugin doesn't show as active after restarting KMail.

EXPECTED RESULT
The settings menu should open for the UMA and the UMA should stay active after program restart.

SOFTWARE/OS VERSIONS
Windows: 
MacOS: 
Linux/KDE Plasma: Manjaro KDE, Kernel 4.19.20-1
(available in About System)
KDE Plasma Version: 5.14.5
KDE Frameworks Version: 5.54.0
Qt Version: 5.12.0

ADDITIONAL INFORMATION
The UMA is still visible, yet unresponsive, regardless of its activation in the plugins menu. When using the command "akonadictl restart," it shows that the plugin crashes repeatedly.

org.kde.pim.akonadicontrol: ProcessControl: Application "/usr/bin/akonadi_unifiedmailbox_agent" stopped unexpectedly ( "Process crashed" )
org.kde.pim.akonadicontrol: Application '/usr/bin/akonadi_unifiedmailbox_agent' crashed too often. Giving up!