Comment 4 for bug 1067265

I get the same error, also I get it quite frequently. Here's an excerpt from my .xsession-errors. It starts like this, and then the errors multiply...

(gnome-shell:4706): Clutter-WARNING **: Actor '<unnamed>[<StBin>:0x3ab7010]' should not be mapped if parent '<notification-container>[<StWidget>:0x37a0fd0]'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 = '2249'
    JS ERROR: !!! stack = '"()@/usr/share/gnome-shell/js/ui/messageTray.js:2249
wrapper()@/usr/share/gjs-1.0/lang.js:204
("_notificationState",2,wrapper,[object Object],(void 0))@/usr/share/gnome-shell/js/ui/messageTray.js:2075
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
(418)@/usr/share/gnome-shell/js/ui/tweener.js:249
wrapper(418)@/usr/share/gjs-1.0/lang.js:204
([object _private_Clutter_Timeline],418)@/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 = '2289'
    JS ERROR: !!! stack = '"()@/usr/share/gnome-shell/js/ui/messageTray.js:2289
wrapper()@/usr/share/gjs-1.0/lang.js:204
()@/usr/share/gnome-shell/js/ui/messageTray.js:1982
wrapper()@/usr/share/gjs-1.0/lang.js:204
()@/usr/share/gnome-shell/js/ui/messageTray.js:1937
wrapper()@/usr/share/gjs-1.0/lang.js:204
"'
    JS ERROR: !!! Exception in callback for signal: showing
    JS ERROR: !!! message = '"this._notification is null"'
    JS ERROR: !!! fileName = '"/usr/share/gnome-shell/js/ui/messageTray.js"'
    JS ERROR: !!! lineNumber = '2289'
    JS ERROR: !!! stack = '"()@/usr/share/gnome-shell/js/ui/messageTray.js:2289
wrapper()@/usr/share/gjs-1.0/lang.js:204
()@/usr/share/gnome-shell/js/ui/messageTray.js:1982
wrapper()@/usr/share/gjs-1.0/lang.js:204
([object Object])@/usr/share/gnome-shell/js/ui/messageTray.js:1507
_emit("showing")@/usr/share/gjs-1.0/signals.js:124
()@/usr/share/gnome-shell/js/ui/overview.js:632
wrapper()@/usr/share/gjs-1.0/lang.js:204
()@/usr/share/gnome-shell/js/ui/overview.js:564
wrapper()@/usr/share/gjs-1.0/lang.js:204