Error "getCurrentSessionProxy() called outside a user session" occurs when starting GNOME

Issue

• We see the following in our message logs when trying to start GNOME:

gnome-shell[2287]: Unset XDG_SESSION_ID, getCurrentSessionProxy() called outside a user session.
[...]
gnome-shell[2287]: JS ERROR: Exception in callback for signal: reset: Gio.DBusError: GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: No session available
                                            _resetGreeterProxy@resource:///org/gnome/shell/gdm/loginDialog.js:827:29
                                            wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
                                            _onReset@resource:///org/gnome/shell/gdm/loginDialog.js:839:9
                                            wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
                                            _emit@resource:///org/gnome/gjs/modules/signals.js:128:27
                                            reset@resource:///org/gnome/shell/gdm/authPrompt.js:621:9
                                            wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
                                            _updateDisableUserList@resource:///org/gnome/shell/gdm/loginDialog.js:746:17
                                            wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
                                            _emit@resource:///org/gnome/gjs/modules/signals.js:128:27
                                            _startupAnimationComplete@resource:///org/gnome/shell/ui/layout.js:708:9
                                            wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
                                            _addHandler/params[name]@resource:///org/gnome/shell/ui/tweener.js:91:13
                                            _callOnFunction@resource:///org/gnome/gjs/modules/tweener/tweener.js:208:13
                                            _updateTweenByIndex@resource:///org/gnome/gjs/modules/tweener/tweener.js:337:9
                                            _updateTweens@resource:///org/gnome/gjs/modules/tweener/tweener.js:350:18
                                            _onEnterFrame@resource:///org/gnome/gjs/modules/tweener/tweener.js:365:10
                                            _emit@resource:///org/gnome/gjs/modules/signals.js:128:27
                                            _onNewFrame@resource:///org/gnome/shell/ui/tweener.js:207:9
                                            wrapper@resource:///org/gnome/gjs/modules/_legacy.js:82:22
                                            _init/<@resource:///org/gnome/shell/ui/tweener.js:182:13