SUMMARY When starting Plasma for the first time (or when applying the Desktop Layout of the Breeze theme), Discover is pinned to the Task Manager even if it's not installed. This results in a generic icon that gives an error when clicked. STEPS TO REPRODUCE 1. Uninstall Discover; 2. Start Plasma for the first time (or Settings>Global Theme>Breeze>Apply Desktop Layout); 3. Look at the Task Manager. OBSERVED RESULT Discover is pinned, despite not being installed. EXPECTED RESULT Discover shouldn't be pinned, or there should be an alternative app (like when Firefox isn't installed, Kate is in its place). SOFTWARE/OS VERSIONS Linux: NixOS 21.11 (available in About System) KDE Plasma Version: 5.25.2 KDE Frameworks Version: 5.95.0 Qt Version: 5.15.3 ADDITIONAL INFORMATION I don't know if this may also apply to the other applications pinned by default, mainly System Settings and Dolphin.
Related to Bug 438611. In this case it's more clear-cut; we should definitely exclude default items that aren't actually installed. That doesn't make any sense. :)
*** This bug has been marked as a duplicate of bug 436667 ***