Error 71 (Protocol error) dispatching to Wayland display

As orinally reported by @mcatanzaro in https://github.com/flatpak/xdg-desktop-portal-gtk/issues/130:

xdg-desktop-portal-gtk-0.11-1.fc28, with gtk3-3.22.30-1.fc28, quits immediately when the file choose portal is used. To reproduce, run /usr/libexec/xdg-desktop-portal-gtk -r, then open Epiphany (as a flatpak) and press Ctrl+O.

Result is: Error 71 (Protocol error) dispatching to Wayland display

According to @jadahl in https://github.com/flatpak/xdg-desktop-portal-gtk/issues/130#issuecomment-386595051, this is probably a bug in mutter, and the error is this:

[4101298.880] wl_display@1.error(zxdg_imported_v1@35, 0, "set_parent_of was called with an invalid child")

Filing the ticket here, and closing the one in the portal