From this morning I had an issue that was getting NULL in $app->handle()->getContent(). After all the day depuring code, I saw that I forgotted that I didn't add a user role that I was using, so I wasn't able to do anything in the app. I think Phalcon would need to throw an exception for this, because he doesn't give you any clue.

private function getACL()
{
    // Stuff.
    foreach ($areas as $controller => $actions)
        {
            $acl->addResource(
                new Resource($controller),
                $actions
            );
            $acl->allow(
                'Anonymous', // <-- This was the user I didn't add in the '$acl->addRole(..)'.
                $controller,
                $actions
            );
        }
    // More stuff.
}