recommended `finish-args`/`--socket` settings?

Since I first packaged by Qt app, even before `com.riverbankcomputing.PyQt.BaseApp`, I've had:

```
finish-args:
- --socket=wayland
- --socket=x11
- --share=ipc
- --device=dri
- --share=network
- --filesystem=home
```

I gather I instead should be using `fallback-x11` (b/c of https://github.com/flathub/org.plomgrading.PlomClient/issues/32 and https://github.com/flathub/org.plomgrading.PlomClient/issues/16)

So I changed it to:
```
finish-args:
- --socket=wayland
- --socket=fallback-x11
- --share=ipc
- --device=dri
- --share=network
- --filesystem=home
```

But now I get:
```
flatpak run org.plomgrading.PlomClient
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: eglfs, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.
```

I guess I can revert my change but I'm very confused by all this socket stuff. for example, this project's example doesn't mention `finish-args:` at all (!)
https://github.com/flathub/com.riverbankcomputing.PyQt.BaseApp/blob/branch/5.15-21.08/examples/PyQtApp.yaml

It would be helpful to have an example of "best practice" into there.