SUMMARY
When xdg-open (or third-party software) invokes opening the file manager (i.e, Dolphin), it results in the following error message:

kf.service.services: KApplicationTrader: mimeType "x-scheme-handler/file" not found


This noticeable in a Qt-based program (XnViewMP) in which not only will it be unable to "open in file manager", it will spit out the above error message in the terminal, as well as a popup message that reads "Error - KIO Client".

Interestingly enough, if I manually invoke,
xdg-open $HOME

It *WILL* open up Dolphin to my Home folder, yet will still spit out the above error message in the terminal.

This makes using third-party apps that open up the file manager break.