[FIX] website: fix ctx argument

In the call to Chart.js we pass a ctx which should be a DOM element,
however we pass `null` as the ctx.

It seems that what happens is that we look for the DOM element inside
the `document` (via getElementById) but this element is not yet inserted
into the document, hence this returns `null`.

Since we already have a reference to the canvas element which should be
used as ctx, we can probably just reuse this reference

closes odoo/odoo#36072

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/website/static/src/js/backend/dashboard.js

@@ -185,7 +185,7 @@ var Dashboard = AbstractAction.extend({
             };
         });
 
-        var ctx = document.getElementById(chart_id);
+        var ctx = this.$canvas[0];
         this.chart = new Chart(ctx, {
             type: 'line',
             data: {

addons/website/static/tests/tours/dashboard_tour.js

@@ -0,0 +1,21 @@
+odoo.define("website.tour.backend_dashboard", function (require) {
+"use strict";
+
+var tour = require("web_tour.tour");
+
+tour.register("backend_dashboard", {
+    test: true,
+    url: "/web",
+}, [tour.STEPS.SHOW_APPS_MENU_ITEM,
+    {
+    trigger: 'a[data-menu-xmlid="website.menu_website_configuration"]',
+    run: 'click',
+}, {
+    trigger: '.dropdown-toggle[data-menu-xmlid="website.menu_dashboard"]',
+    run: 'click',
+}, {
+    trigger: '.dropdown-item[data-menu-xmlid="website.menu_website_google_analytics"]',
+    content: 'Check if traceback',
+    run: 'click',
+}]);
+});

addons/website/tests/test_ui.py

@@ -109,3 +109,6 @@ class TestUi(odoo.tests.HttpCase):
                 ])]
         })
         self.start_tour("/", 'restricted_editor', login='restricted')
+
+    def test_04_backend_dashboard(self):
+        self.start_tour("/", 'backend_dashboard', login='admin')

addons/website/views/website_templates.xml

@@ -23,6 +23,7 @@
         <script type="text/javascript" src="/website/static/tests/tours/rte.js"/>
         <script type="text/javascript" src="/website/static/tests/tours/html_editor.js"/>
         <script type="text/javascript" src="/website/static/tests/tours/restricted_editor.js"/>
+        <script type="text/javascript" src="/website/static/tests/tours/dashboard_tour.js"/>
     </xpath>
 </template>