This seems to have appeared in recent trunk, just by starting Thunderbird. Contrary to bug 632174, this happens only when the event search widget is enabled (but the calendar tab may be closed). Error: TypeError: this.tree is null Source File: chrome://calendar/content/calendar-unifinder.js Line: 513 The code is: setItems: function uTV_setItems(aItemArray, aDontSort) { let oldCount = this.eventArray.length; this.eventArray = aItemArray.slice(0); this.tree.rowCountChanged(oldCount - 1 , (this.eventArray.length - oldCount)); Actual results: Error: [Exception... "'[JavaScript Error: "this.tree is null" {file: "chrome://calendar/content/calendar-unifinder.js" line: 501}]' when calling method: [calIObserver::onAddItem]" nsresult: "0x80570021 (NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS)" location: "JS frame :: file:///.../calendar-js/calUtils.js :: notifyFunc :: line 1227" data: yes] STACK: 1: [file:///.../calendar-js/calUtils.js:1230] notifyFunc 2: [file:///.../calendar-js/calUtils.js:1233] calListenerBag_notify 3: [file:///.../modules/calProviderUtils.jsm:497] calObserverBag_notify 4: [file:///.../components/calCompositeCalendar.js:78] anonymous 5: [null:0] null 6: [file:///.../calendar-js/calUtils.js:1227] notifyFunc 7: [file:///.../calendar-js/calUtils.js:1233] calListenerBag_notify 8: [file:///.../modules/calProviderUtils.jsm:497] calObserverBag_notify 9: [file:///.../calendar-js/calStorageCalendar.js:455] cSC_adoptItem 10: [file:///.../calendar-js/calStorageCalendar.js:403] cSC_addItem Source file: file:///.../calendar-js/calUtils.js Line: 1230