Unable to run Python demos with SourceView in the ui file

[UrtsiSantsi]

(process:101): Gtk-ERROR **: 01:30:56.799: failed to add UI: .:8:1 Invalid object type 'GtkSourceView'

[theCapypara]

I can't reproduce this, are you using the Flatpak based development environment and did you import GtkSource?

import gi
gi.require_version("GtkSource", "5")
from gi.repository import GtkSource

print(GtkSource.View)

prints

<class 'gi.repository.GtkSource.View'>

[UrtsiSantsi]

I'm using Gnome Builder 45 from Flathub on Fedora 38. How do I get a "Flatpak based development environment" in general?
I tried with importing it and with a just a single line - print("Something") - in the demo, the result is the same.

[theCapypara]

If you are running Workbench via GNOME Builder it will use the Flatpak.

That is odd, so with that exact snippet I shared you get an error?

[UrtsiSantsi]

Yes, even with the exact same snippet. Here is the Blueprint:

using Gtk 4.0;
using GtkSource 5;

Box {
  $GtkSourceView source_view {
  }
}

Maybe there is something wrong with my Builder?

[theCapypara]

Why do you have a $ in front of GtkSourceView? I'm pretty sure it should be just Gtk.SourceView, also note the ..

[sonnyp]

You should be using GtkSource.View not $GtkSourceView.

Possibly GtkSource.init should be called in Python

[UrtsiSantsi]

I switched Blueprint -> XML -> Blueprint as a way to format the Blueprint and I guess it changed it somehow. But even when it is corrected it still doesn't work. I also tried GtkSource.init without success.
At some point it started working somehow, but other things didn't - like some of the Adwaita elements were missing. Now it doesn't work again, without any changes in the code.
I'm trying to port the "CSS Gradients" demo and I pushed #724 - could you check it.