Thanks to this "How can I have latest Dolphin file manager on Ubuntu" I was able to install Dolphin [flatpk] on Ubuntu 19.04. However if I click on any entry like "Documents", "Pictures", etc. on Arc Menu Nautilus opens. I am not able to change that behavior to Dolphin..
I tried to alter this common script to match with Dolphin, but with no success:
xdg-mime default nemo.desktop inode/directory application/x-gnome-saved-search
Finally it was pretty easy:
To get the application name of Dolphin:
flatpak list
Description Application Version Zweig Arch Origin Installation
Dolphin - Filemanager org.kde.dolphin stable x86_64 flathub system
Change default filemanager:
xdg-mime default org.kde.dolphin.desktop inode/directory application/x-gnome-saved-search
My question is similar to Flatpak applications use nautilus instead of Dolphin, but that got no real answer IMO.
If you want to get to know more about Flatpak file paths: Link