[FIX] website: fix background video widget on mobile

Issue

    - Download the customer db: stargue-inc
    - Go on the website in mobile mode

    Traceback & the second item in the carousel is not displayed.

Cause

    Traceback: we use `this.$iframe` without checking if it's defined
    Second item not displayed: the first video promise is overriden
    by the second video promise.

Solution

    Check if this.$iframe is defined and resolve the old promise
    instead of overriding it.

OPW-2152690

closes odoo/odoo#42139

X-original-commit: f5f569b4705f37865cb976a4df10a0d9f388fae2
Signed-off-by: Jason Van Malder <jvm-odoo@users.noreply.github.com>

addons/website/static/src/js/content/snippets.animation.js

@@ -664,8 +664,14 @@ registry.backgroundVideo = publicWidget.Widget.extend({
             this.videoSrc = this.videoSrc + "&enablejsapi=1";
 
             if (!window.YT) {
+                var oldOnYoutubeIframeAPIReady = window.onYouTubeIframeAPIReady;
                 proms.push(new Promise(resolve => {
-                    window.onYouTubeIframeAPIReady = () => resolve();
+                    window.onYouTubeIframeAPIReady = () => {
+                        if (oldOnYoutubeIframeAPIReady) {
+                            oldOnYoutubeIframeAPIReady();
+                        }
+                        return resolve();
+                    };
                 }));
                 $('<script/>', {
                     src: 'https://www.youtube.com/iframe_api',
@@ -713,6 +719,10 @@ registry.backgroundVideo = publicWidget.Widget.extend({
      * @private
      */
     _adjustIframe: function () {
+        if (!this.$iframe) {
+            return;
+        }
+
         this.$iframe.removeClass('show');
 
         // Adjust the iframe