New here! I need some help for my first flatpak package

Hi everyone. I’ve been using Flatpak for a while now, and liked it. Now I’m trying to create a Linphone package (its devs used to provide a remote flatpak repository, but now it’s gone) based on the AppImage (yeah, I’m just too lazy to build from source, and its dependencies).

I’ve seen some packages that build AppImages, and I successfully built one, but for some reason when trying to run the app I get this error:

qt.qpa.xcb: could not connect to display 
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: xcb.

This is the yml file. I run flatpak-builder --user --install --force-clean ../linphone-build-dir com.belledonnecommunications.linphone.yml, and then when doing flatpak run com.belledonnecommunications.linphone I get that error.

I’m sure that the solution might be easy, but I’m a complete noob in flatpak packaging, that’s why I came here!
Thanks in advance.

You should use KDE’s SDK/Runtime if you’re using Qt.