Controller actions can only return ResponseInterface instance or null. Got App\Model\Entity\Tvchannel instead. 📋

UnexpectedValueException
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    public function invokeAction(Closure $action, array $args): void
    {
        $result $action(...$args);
        if ($result !== null && !$result instanceof ResponseInterface) {
            throw new UnexpectedValueException(sprintf(
                'Controller actions can only return ResponseInterface instance or null. '
                'Got %s instead.',
                getTypeName($result)
            ));

If you want to customize this error message, create templates/Error/error500.php