sideload failed to install from flatpakref file

### What Happened?

**Note:** I already described the bug at GNOME [Gitlab](https://gitlab.gnome.org/GNOME/epiphany/-/issues/2078), but in brief:

I was trying to install Epiphany Tech Preview, to understand which bugs to report to elementary and which bugs should go upstream. I downloaded the [flatpakref file](https://nightly.gnome.org/repo/appstream/org.gnome.Epiphany.Devel.flatpakref) linked in their README. After opening the downloaded file and confirming "I understand", I proceeded to meet the following error (translated from Czech):

```
Cannot find ref „app/org.gnome.Epiphany.Devel/x86_64/master“ in remote repository gnome-nightly
```

Redoing the same operation from terminal succeeded without problems, using `flatpak install org.gnome.Epiphany.Devel.flatpakref`.

### Steps to Reproduce

1. Download the flatpakref file of Epiphany Tech Preview (uses GNOME Nightly repo, which was not previously set up on my system).
2. Open the downloaded file with Sideload.
3. Confirm and proceed.
4. See error.

### Expected Behavior

No error, it should just work.

### OS Version

7.x (Horus)

### Software Version

Latest release (I have run all updates)

### Log Output

_No response_

### Hardware Info

Slimbook Essential 14.