TYPO3 Core

I am using imports in the site configuration's config.yaml like this:

imports:
  - { resource: 'EXT:myext1/Configuration/Sites/config.yaml' }
  - { resource: 'EXT:myext2/Configuration/Sites/config.yaml' }
  - { resource: 'EXT:myext3/Configuration/Sites/config.yaml' }

When I flush the cache (or remove var/cache completely) and make also a clean install of composer dependencies the following exception is thrown:

Call to a member function isPackageActive() on null
in /var/www/html/public/typo3/sysext/core/Classes/Utility/ExtensionManagementUtility.php line 120

at TYPO3\CMS\Core\Utility\ExtensionManagementUtility::isLoaded('myext1')
in /var/www/html/public/typo3/sysext/core/Classes/Utility/GeneralUtility.php line 2816

at TYPO3\CMS\Core\Utility\GeneralUtility::getFileAbsFileName('')
in /var/www/html/public/typo3/sysext/core/Classes/Configuration/Loader/YamlFileLoader.php line 125

at TYPO3\CMS\Core\Configuration\Loader\YamlFileLoader->getStreamlinedFileName('EXT:myext1/Configuration/Sites/config.yaml', '/var/www/html/config/sites/main/config.yaml')
in /var/www/html/public/typo3/sysext/core/Classes/Configuration/Loader/YamlFileLoader.php line 81

at TYPO3\CMS\Core\Configuration\Loader\YamlFileLoader->loadAndParse('EXT:myext1/Configuration/Sites/config.yaml', '/var/www/html/config/sites/main/config.yaml')
in /var/www/html/public/typo3/sysext/core/Classes/Configuration/Loader/YamlFileLoader.php line 164