Javascript error is report to the JS console (tested in Chrome and Edge) when the exception is handled by Symfony Profiler:

Uncaught TypeError: Cannot set property 'className' of null at Object.createTabs

May be related to #24281.

Steps to reproduce:

1. Create a new flex project

composer create-project symfony/skeleton blog
cd blog
composer require server --dev
composer require profiler --dev 

2. Create a controller and enable its route in config/routes.yaml:

<?php
namespace App\Controller;

class DefaultController {
	public function index()
	{
		throw new \Exception('Wow!');
	}
}

3. Run php bin\console server:run
4. Open the browser
5. Info about the exception is rendered properly
6. But there is an error in the browser JS console