Cant access to extension settings.

[MarcosLans]

Describe the Bug

The extension works fine for me but as I try to acces to settings extension I get an error.

Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

You may also check the output of GNOME Shell for any error messages.
This can be done with the following terminal command:

journalctl -f -o cat | grep -E 'burn-my-windows|'

## Expected Behavior
Extension works well for me but when I try access to settings I got an error (anexed screenshot)

## System
_Please complete the following information:_
 - Linux distribution : Ubuntu 20.04.3 LTS
 - Burn-My-Windows version : I cant find it.
 - GNOME Shell version: 3.36.8

log: 
The settings of extension burn-my-windows@schneegans.github.com had an error:

Gtk.BuilderError: .:12:43 Invalid property: GtkViewport.scroll-to-focus

Stack trace:

PreferencesDialog@/home/brath/.local/share/gnome-shell/extensions/burn-my-windows@schneegans.github.com/prefs.js:37:19
buildPrefsWidget@/home/brath/.local/share/gnome-shell/extensions/burn-my-windows@schneegans.github.com/prefs.js:99:16
_init@resource:///org/gnome/Shell/Extensions/js/extensionsService.js:207:40
OpenExtensionPrefsAsync/<@resource:///org/gnome/Shell/Extensions/js/extensionsService.js:122:28
asyncCallback@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:132:13
run@resource:///org/gnome/Shell/Extensions/js/dbusService.js:175:20
main@resource:///org/gnome/Shell/Extensions/js/main.js:19:13
run@resource:///org/gnome/gjs/modules/script/package.js:222:19
start@resource:///org/gnome/gjs/modules/script/package.js:206:5
@/usr/share/gnome-shell/org.gnome.Shell.Extensions:1:17

![Captura de pantalla de 2021-12-15 17-49-14 png](https://user-images.githubusercontent.com/11011882/146230954-d4e96915-3642-41b3-9ce5-7bcee25184ad.png)

[MarcosLans]

journalctl -f -o cat | grep -E 'burn-my-windows|'

Window manager warning: Buggy client sent a _NET_ACTIVE_WINDOW message with a timestamp of 0 for 0x3000012
Window manager warning: Ping serial 68601535 was reused for window 0x2c0002c, previous use was for window 0x2c19789.
systemd-hostnamed.service: Succeeded.
Started Application launched by gnome-shell.
[session uid=1000 pid=1431] Activating via systemd: service name='org.gnome.Terminal' unit='gnome-terminal-server.service' requested by ':1.1330' (uid=1000 pid=69036 comm="/usr/bin/gnome-terminal.real --window " label="unconfined")
Starting GNOME Terminal Server...
[session uid=1000 pid=1431] Successfully activated service 'org.gnome.Terminal'
Started GNOME Terminal Server.
Started VTE child process 69048 launched by gnome-terminal-server process 69040.
gnome-launched-org.gnome.Terminal.desktop-69033.scope: Succeeded.

[Schneegans]

Yeah, I know that it mostly works on GNOME Shell 3.36 and 3.38, but the settings dialog is currently GTK4-only. So it will not work on GNOME 3.3x. If I have the time, I may back-port it, but currently the extension only supports GNOME Shell 40 and 41.

[Schneegans]

I just released Burn-My-Windows 3 which adds official support for GNOME 3.36 and 3.38.