[FIX] mail: no traceback on receiving seen notification

Before this commit, when we post a message and someone
has received it, there could be a traceback.

Steps to reproduce:
1. Two users are logged in.
2. On one user:
   - open any app except Discuss
   - From systray messaging menu, click on "New messages"
   - Open Discuss
   - Post a message in a direct chat with other User.
> traceback `Cannot read property 'getID' of null`

closes odoo/odoo#31228

addons/mail/static/src/js/thread_windows/thread_window.js

@@ -377,6 +377,9 @@ var ThreadWindow = AbstractThreadWindow.extend({
      * @param {integer} channelID
      */
     _onUpdateChannel: function (channelID) {
+        if (!this.hasThread()) {
+            return;
+        }
         if (this._thread.getID() !== channelID) {
             return;
         }

addons/mail/static/tests/systray/systray_messaging_menu_tests.js

@@ -634,6 +634,50 @@ QUnit.test("messaging menu widget: channel seen notification", function (assert)
     messagingMenu.destroy();
 });
 
+QUnit.test("messaging menu widget: no traceback when receiving channel_fetched notification with blank thread window", function (assert) {
+    assert.expect(3);
+
+    this.data.initMessaging.channel_slots = {
+        channel_direct_message: [{
+            id: 1,
+            channel_type: "chat",
+            direct_partner: [{ id: 2, name: 'Someone', im_status: '' }],
+            name: 'DM',
+            message_unread_counter: 1,
+        }],
+    };
+
+    var messagingMenu = new MessagingMenu();
+    testUtils.mock.addMockEnvironment(messagingMenu, {
+        services: this.services,
+        data: this.data,
+        session: { partner_id: 3 },
+    });
+
+    messagingMenu.appendTo($('#qunit-fixture'));
+    messagingMenu.$('.dropdown-toggle').click();
+    assert.containsOnce(messagingMenu, '.o_new_message',
+        "should have button to open blank thread window");
+
+    testUtils.dom.click(messagingMenu.$('.o_new_message'));
+    var $threadWindow = $('.o_thread_window');
+    assert.strictEqual($threadWindow.length, 1, "should have an open thread window");
+    assert.ok($threadWindow.hasClass('o_thread_less'), "should be a blank thread window");
+
+    // Simulate received channel fetched notification
+    var message = {
+        info: 'channel_fetched',
+        last_message_id: 1,
+        partner_id: 2,
+    };
+    var notifications = [
+        [['myDB', 'mail.channel', 1], message]
+    ];
+    messagingMenu.call('bus_service', 'trigger', 'notification', notifications);
+
+    messagingMenu.destroy();
+});
+
 QUnit.test("messaging menu widget: preview with no message should be undated", function ( assert ) {
     assert.expect(2);