Failed to load module "pantheon-filechooser-module"

## Description

I see the following message when starting the application from command line:

```
$ flatpak run com.github.maoschanz.drawing
Gtk-Message: 01:27:30.708: Failed to load module "pantheon-filechooser-module"
```

## Steps to reproduce the bug:

1. Install app from command line using `flatpak install flathub com.github.maoschanz.drawing`
2. Run app from command line using `flatpak run com.github.maoschanz.drawing`

## Expected behavior

App running without any error messages.

## System

- Device (desktop computer/tablet/phone/...): notebook
- OS: elementary OS 5.0 Juno
- Package format (flatpak, DEB, AUR ?): flatpak
- App version: 0.2.0

Reproduce the warning with the documented flatpak install and run commands on the reported environment, then investigate the application startup path and Flatpak runtime configuration. Done means the application starts without the reported "pantheon-filechooser-module" message.

Written by the indexing model from the issue text.