Exception upon clearing system cache

[buepro]

When using the extension in TYPO3 8.7.18 and clearing the system cache an exception occures. Deleteing the cache files doesn't help. The issue seems to be related to the cache configuration in ext_localconf.php. When commenting the related lines the cache can be cleared:

// cache configuration, see
// https://docs.typo3.org/typo3cms/...
//$TYPO3_CONF_VARS['SYS']['caching']['cacheConfigurations']['rssdisplay']['frontend']...
//$TYPO3_CONF_VARS['SYS']['caching']['cacheConfigurations']['rssdisplay']['groups']...

The test environment was using the distribution pizpalue which uses the bootstrap_package.