Bug 1665831 - Fix two calendar UI start-up errors. r=pmorris

Differential Revision: https://phabricator.services.mozilla.com/D90650

--HG--
extra : moz-landing-system : lando

calendar/base/content/calendar-tabs.js

@@ -25,7 +25,7 @@ var calendarTabMonitor = {
     // type definitions. To make sure the commands are correctly disabled,
     // we want to update calendar/task commands when switching away from
     // those tabs.
-    if (aOldTab.mode.name == "calendar" || aOldTab.mode.name == "task") {
+    if (aOldTab?.mode.name == "calendar" || aOldTab?.mode.name == "task") {
       calendarController.updateCommands();
       calendarController2.updateCommands();
     }

calendar/base/content/calendar-unifinder.js

@@ -43,7 +43,7 @@ var gUnifinderNeedsRefresh = true;
 function isUnifinderHidden() {
   let tabmail = document.getElementById("tabmail");
   return (
-    tabmail.currentTabInfo.mode.type != "calendar" ||
+    tabmail.currentTabInfo?.mode.type != "calendar" ||
     document.getElementById("bottom-events-box").hidden
   );
 }