🐛 amp-access-poool: Ensure main container exists (#26939)

* fix(amp-access-poool): ensure container exists

Fixes #26893

* fix(amp-access-poool): upgrade legacy api url

Co-authored-by: Nicolas AUGER <nicolas@poool.fr>

extensions/amp-access-poool/0.1/poool-impl.js

@@ -15,7 +15,7 @@
  */
 import {Services} from '../../../src/services';
 import {addParamToUrl, addParamsToUrl} from '../../../src/url';
-import {dev, userAssert} from '../../../src/log';
+import {dev, user, userAssert} from '../../../src/log';
 import {dict} from '../../../src/utils/object';
 import {getMode} from '../../../src/mode';
 import {listenFor} from '../../../src/iframe-helper';
@@ -24,7 +24,7 @@ import {resetStyles, setStyle, setStyles} from '../../../src/style';
 const TAG = 'amp-access-poool';
 
 const ACCESS_CONFIG = {
-  'authorization': 'https://api.poool.fr/api/v2/amp/access?rid=READER_ID',
+  'authorization': 'https://api.poool.fr/api/v3/amp/access?rid=READER_ID',
   'iframe':
     'https://assets.poool.fr/amp.html' +
     '?rid=READER_ID' +
@@ -174,11 +174,23 @@ export class PooolVendor {
       });
   }
 
+  /**
+   * @return {!Element}
+   * @private
+   */
+  getContainer_() {
+    const paywallContainer = this.ampdoc.getElementById('poool');
+    return user().assertElement(
+      paywallContainer,
+      'No element with id #poool found to render paywall into, got'
+    );
+  }
+
   /**
    * @private
    */
   renderPoool_() {
-    const pooolContainer = this.ampdoc.getElementById('poool');
+    const pooolContainer = this.getContainer_();
     const urlPromise = this.accessSource_.buildUrl(
       addParamsToUrl(
         this.iframeUrl_,