Ubuntu 18.04: Nvidia + Flatpak not working

I upgraded from 16.04 and then installed this https://flatpak.org/setup/Ubuntu/ - It works to find and install flatpaks from the ubuntu software center, but no flatpak app works to launch.

To give you an example of the error, here's what I get from Kdenlive:

tio@tio:~$ flatpak run org.kde.kdenlive
ERROR: ld.so: object 'libgtk3-nocsd.so.0' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
Gtk-Message: Failed to load module "canberra-gtk-module"
Gtk-Message: Failed to load module "canberra-gtk-module"
libGL error: No matching fbConfigs or visuals found
libGL error: failed to load driver: swrast
Could not initialize GLX

I have Nvidia 396.24.02 installed. Thing is, if I switch to X.Org (Nouveau) they work just fine. Problem is, with X.Org I get a laggy desktop environment. So I understand that Flatpak doesn't like Nvidia? Is there any way to make these work with Nvidia?

I searched and searched and didn't find any answer.

EDIT: It seems the issue is with graphical tools like Kdenlive or Krita. Not with apps like Riot or Signal. I suppose the graphical tools need access to Nvidia and they can't find it?! I hope someone can help. Thanks!

EDIT 2: GIMP for example works. Riot works, Signal works, and most flatpak work. Kdenlive and Krita do not. Same error for both. Very strange.

EDIT 3: I have a better answer after investigating. It seems some packages were missing from the Nvidia install so I installed Kdelive from the flathub website: flatpak run org.kde.kdenlive. That installed some flatpak dependencies, and after Kdenlive was successfully installed all the other apps that didn't work, worked again.