[FIX] demo: optional askConfirmation cancel argument

The argument `cancel` of the method `askConfirmation` is optional,
but the function `cancel` is always called in the demo, without checking
if its defined. this lead to tracebacks if `cancel` is not defined.

closes #2274

X-original-commit: d91bf98
Signed-off-by: Pierre Rousseau (pro) <pro@odoo.com>
Signed-off-by: Minne Adrien (adrm) <adrm@odoo.com>

demo/main.js

@@ -203,7 +203,7 @@ class Demo extends Component {
     if (window.confirm(content)) {
       confirm();
     } else {
-      cancel();
+      cancel?.();
     }
   }