[FIX] website: fix traceback on refresh

Commit [1] made it possible to stay in the backend while refreshing the
page with F5 or CTRL+R when viewing a Website Preview. Pressing it too
fast when the page is still loading and the iframe isn't loaded yet
triggers a traceback.
This commit falls back to the default refresh in such cases.

[1]: https://github.com/odoo/odoo/commit/e69c6eaed4e82e08d6bbf807cf4698f6327a9cdd

task-3795143

closes odoo/odoo#158314

X-original-commit: c81dfa7dda95a8d63b4abba9ce67d7f3e48af935
Signed-off-by: Arthur Detroux (ard) <ard@odoo.com>
Signed-off-by: Robin Lejeune (role) <role@odoo.com>

addons/website/static/src/client_actions/website_preview/website_preview.js

@@ -553,6 +553,10 @@ export class WebsitePreview extends Component {
         if (hotkey !== 'control+r' && hotkey !== 'f5') {
             return;
         }
+        // The iframe isn't loaded yet: fallback to default refresh.
+        if (this.websiteService.contentWindow === undefined) {
+            return;
+        }
         ev.preventDefault();
         const path = this.websiteService.contentWindow.location;
         const debugMode = this.env.debug ? `?debug=${this.env.debug}` : "";