Bug 1773972 - Check if folderDisplay.view.dbView is null before using it. r=mkmelin

Differential Revision: https://phabricator.services.mozilla.com/D154501

--HG--
extra : amend_source : 070920242881ba8cf7ae1a9841b6a7cb449a3934
extra : absorb_source : 893ff6abad05ae9fcb3c91fe673f9832adae811d

mail/components/extensions/parent/ext-mailTabs.js

@@ -310,7 +310,9 @@ this.mailTabs = class extends ExtensionAPI {
         async getSelectedMessages(tabId) {
           let tab = getTabOrActive(tabId);
           let { folderDisplay } = tab.nativeTab;
-          let messageList = folderDisplay.view.dbView.getSelectedMsgHdrs();
+          let messageList = folderDisplay.view.dbView
+            ? folderDisplay.view.dbView.getSelectedMsgHdrs()
+            : [];
           return messageListTracker.startList(messageList, extension);
         },