XML dialog empty on open (and stops appearing once closed)

Summary:

With Inkscape 1.5-dev, opening the XML dialog opens an empty window initially. Once this is closed, trying to open the dialog does nothing. The behaviour is similar to the Preferences dialog's - however inkscape#4802 only mentions some missing delete icons... Regression from Gtk3.

Steps to reproduce:

• open Inkscape after clearing preferences
• open the XML editor
• close the window
• try opening the XML editor

What happened?

Blank Window opens at step 2, nothing at 4

What should have happened?

XML dialog should have opened.

Version info

Inkscape 1.5-dev (afb39433d1, 2024-05-12, custom)

                      Compile  (Run)
    GLib version:     2.78.3
    GTK version:      4.14.2 (4.14.2)
    glibmm version:   2.78.1
    gtkmm version:    4.14.0
    libxml2 version:  2.10.4
    libxslt version:  1.1.39
    Cairo version:    1.18.0 (1.18.0)
    Pango version:    1.51.0 (1.51.0)
    HarfBuzz version: 8.2.1 (8.2.1)

    OS version:       Fedora Linux 39 (Workstation Edition)