On fresh Leap 15.6 beta install from ISO image:
 
After setting the dark theme in the GNOME environment settings, there is a GNOME Shell crash during the GNOME session user login. The following messages are visible in the journald log:

mar 08 21:24:43 localhost.localdomain gnome-shell[2381]: JS ERROR: Gio.IOErrorEnum: Error: GSettings key picture-uri-dark not found in schema org.gnome.desktop.background.lockdialog
                                                         
                                                         Stack trace:
                                                           _checkKey@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:779:23
                                                           createCheckedMethod/<@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:729:30
                                                           getBackground@resource:///org/gnome/shell/ui/background.js:603:44
                                                           _createBackgroundActor@resource:///org/gnome/shell/ui/background.js:792:49
                                                           BackgroundManager@resource:///org/gnome/shell/ui/background.js:718:37
                                                           _createBgLockDialogGroup@resource:///org/gnome/shell/ui/screenShield.js:556:25
                                                           _updateBgLockDialogGroup@resource:///org/gnome/shell/ui/screenShield.js:589:18
                                                           ScreenShield@resource:///org/gnome/shell/ui/screenShield.js:89:14
                                                           _initializeUI@resource:///org/gnome/shell/ui/main.js:232:24
                                                           start@resource:///org/gnome/shell/ui/main.js:176:11
                                                           @resource:///org/gnome/shell/ui/init.js:12:47
                                                           @resource:///org/gnome/shell/ui/init.js:21:20
                                                           
                                                         @resource:///org/gnome/shell/ui/init.js:21:20
mar 08 21:24:43 localhost.localdomain gnome-shell[2381]: Execution of main.js threw exception: Module resource:///org/gnome/shell/ui/init.js threw an exception
mar 08 21:24:43 localhost.localdomain gnome-shell[2381]: double free or corruption (out)
mar 08 21:24:43 localhost.localdomain gnome-shell[2381]: == Stack trace for context 0x55b6c9f1e450 ==
mar 08 21:24:43 localhost.localdomain systemd[2142]: org.gnome.Shell@wayland.service: Main process exited, code=dumped, status=6/ABRT
mar 08 21:24:43 localhost.localdomain systemd[2142]: org.gnome.Shell@wayland.service: Failed with result 'core-dump'.

To restore the ability to log in, after switching to the text console, I executed the following commands:

dconf reset /org/gnome/desktop/interface/color-scheme
dconf reset /org/gnome/desktop/background/picture-uri-dark
dconf reset /org/gnome/desktop/background/picture-uri
sudo systemctl restart display-manager.service