Message tray disappears in some conditions

I did not fully understand what actions cause message tray disappears. In most cases it disappears when few messages from different buddies was received in one time. Also sometimes tray dissapears when screen is locked and there was recived few messages from two or more buddies.

When message tray dissapears in session.log appears next lines

(gnome-shell:3573): Clutter-WARNING **: Actor '<unnamed>[<StBin>:0x9e6b9d0]' should not be mapped if parent '<notification-container>[<StWidget>:0x9ad16e8]'is not visible
    JS ERROR: !!! Error calling onComplete
    JS ERROR: !!! message = '"this._notification is null"'
    JS ERROR: !!! fileName = '"/usr/share/gnome-shell/js/ui/messageTray.js"'
    JS ERROR: !!! lineNumber = '2240'
    JS ERROR: !!! stack = '"()@/usr/share/gnome-shell/js/ui/messageTray.js:2240
wrapper()@/usr/share/gjs-1.0/lang.js:204
("_notificationState",2,wrapper,[object Object],(void 0))@/usr/share/gnome-shell/js/ui/messageTray.js:2063
wrapper("_notificationState",2,wrapper,[object Object],(void 0))@/usr/share/gjs-1.0/lang.js:204
("_notificationState",2,wrapper,[object Object],(void 0))@/usr/share/gnome-shell/js/ui/tweener.js:109
_callOnFunction((function () {oldHandler.apply(eventScope, oldParams);handler(target);}),"onComplete",[object Object],[object _private_St_Widget],[object Array])@/usr/share/gjs-1.0/tweener/tweener.js:202
_updateTweenByIndex(0)@/usr/share/gjs-1.0/tweener/tweener.js:332
_updateTweens()@/usr/share/gjs-1.0/tweener/tweener.js:344
_onEnterFrame([object Object])@/usr/share/gjs-1.0/tweener/tweener.js:359
_emit("prepare-frame")@/usr/share/gjs-1.0/signals.js:124
(405)@/usr/share/gnome-shell/js/ui/tweener.js:249
wrapper(405)@/usr/share/gjs-1.0/lang.js:204
([object _private_Clutter_Timeline],405)@/usr/share/gnome-shell/js/ui/tweener.js:224
"'
    JS ERROR: !!! Exception was: TypeError: this._notification is null
    JS ERROR: !!! message = '"this._notification is null"'
    JS ERROR: !!! fileName = '"/usr/share/gnome-shell/js/ui/messageTray.js"'
    JS ERROR: !!! lineNumber = '2280'
    JS ERROR: !!! stack = '"()@/usr/share/gnome-shell/js/ui/messageTray.js:2280
wrapper()@/usr/share/gjs-1.0/lang.js:204
()@/usr/share/gnome-shell/js/ui/messageTray.js:1970
wrapper()@/usr/share/gjs-1.0/lang.js:204
()@/usr/share/gnome-shell/js/ui/messageTray.js:1925
wrapper()@/usr/share/gjs-1.0/lang.js:204
"'

ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: gnome-shell 3.6.2-0ubuntu0.2
ProcVersionSignature: Ubuntu 3.5.0-19.30-generic 3.5.7
Uname: Linux 3.5.0-19-generic i686
ApportVersion: 2.6.1-0ubuntu6
Architecture: i386
CheckboxSubmission: 2ea33190e5f581bedd2dbf14be2f26be
CheckboxSystem: 4ed15c40009aa6f7770f606350a390a2
Date: Fri Nov 30 13:50:31 2012
DisplayManager: gdm
MarkForUpload: True
ProcEnviron:
 LANGUAGE=ru:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=ru_RU.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-shell
UpgradeStatus: Upgraded to quantal on 2012-11-01 (28 days ago)