SUMMARY
Plugins which wants to install their own schemes (KDE integration with KIO) needs to be preloaded before any QWebEngine class is created.
See docs for more details:
https://doc.qt.io/qt-5/qwebengineurlscheme.html#details


STEPS TO REPRODUCE
1. Start Falkon with KDE integration plugin enabled or enable this plugin later
2. Check the console output

OBSERVED RESULT
QWebEngineUrlScheme::registerScheme() before installing the custom scheme handler.
Please register the custom scheme '<name>' via QWebEngineUrlScheme::registerScheme() before installing the custom scheme handler.

EXPECTED RESULT
Schemes are registered and works, no more warning messages.

SUGGESTION:
I would add new variable into .desktop (basic plugin information) file called "needPreload" or something similar.
Cut the loading code of plugins into more parts which will be called separately.
One to load and run preload() before almost anything is ready and second after things are ready (at current position).

The last option is to register all possible schemes which is a waste.
Also we can consider adding a field to .desktop file in which we would specify the schemes, but this is not cumbersome and would require a lot of work especially when it come to KIO schemed which can be anything.

SOFTWARE/OS VERSIONS
Qt Version: 5.14

ADDITIONAL INFORMATION
Also some internal parts needs to be modified.
Plugins loaded while Falkon is running will not be able to register custom scheme.