SonataAdminBundle: PHP Fatal error: Uncaught Symfony\Component\Debug\Exception\ContextErrorException: Warning: array_merge(): Argument #2 is not an array

Sonata packages

$ composer show --latest 'sonata-project/*'

sonata-project/admin-bundle 3.23.0 3.23.0 The missing Symfony Admin Generator
sonata-project/block-bundle 3.2.0 3.4.0 Symfony SonataBlockBundle sonata-project/cache 1.0.0 1.0.7 Cache library sonata-project/core-bundle 3.4.0 3.5.1 Symfony SonataCoreBundle sonata-project/datagrid-bundle 2.2 2.2.1 Symfony SonataDatagridBundle sonata-project/doctrine-orm-admin-bundle 3.1.0 3.1.7 Symfony Sonata / Integrate Doctrine ORM into the SonataAdminBundle sonata-project/exporter 1.7.0 1.7.1 Lightweight Exporter library

Symfony packages

$ composer show --latest 'symfony/*'

symfony/monolog-bundle v3.1.0 v3.1.1 Symfony MonologBundle
symfony/phpunit-bridge v3.3.0 v3.3.9 Symfony PHPUnit Bridge symfony/polyfill-apcu v1.1.0 v1.5.0 Symfony polyfill backporting apcu_* functions to lower PHP versions symfony/polyfill-intl-icu v1.0.0 v1.5.0 Symfony polyfill for intl’s ICU-related data and classes symfony/polyfill-mbstring v1.0.0 v1.5.0 Symfony polyfill for the Mbstring extension symfony/polyfill-php56 v1.0.0 v1.5.0 Symfony polyfill backporting some PHP 5.6+ features to lower PHP versions symfony/polyfill-php70 v1.0.0 v1.5.0 Symfony polyfill backporting some PHP 7.0+ features to lower PHP versions symfony/polyfill-util v1.0.0 v1.5.0 Symfony utilities for portability of PHP codes symfony/security-acl v3.0.0 v3.0.0 Symfony Security Component - ACL (Access Control List) symfony/swiftmailer-bundle v2.3.10 v3.0.3 Symfony SwiftmailerBundle symfony/symfony v3.3.0 v3.3.9 The Symfony PHP framework

PHP version

$ php -v
PHP 7.1.10-1+0~20170929170818.9+stretch~1.gbp501135 (cli) (built: Sep 29 2017 17:08:20) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.1.10-1+0~20170929170818.9+stretch~1.gbp501135, Copyright (c) 1999-2017, by Zend Technologies

Subject

PHP Fatal error: Uncaught Symfony\Component\Debug\Exception\ContextErrorException: Warning: array_merge(): Argument #2 is not an array