I am attempting to upgrade from 2.8.20 -> 2.8.21 on composer update, I get this exception:

post-install-cmd: Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache
PHP Fatal error: Call to a member function getContainer() on null in /Applications/MAMP/htdocs/core.git/src/lib/Zikula/Bundle/CoreBundle/Console/Application.php on line 44
Script Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache handling the post-install-cmd event terminated with an exception

[RuntimeException]
An error occurred when executing the "'cache:clear --no-warmup'" command:
PHP Fatal error: Call to a member function getContainer() on null in /Applications/MAMP/htdocs/core.git/src/lib/Zikula/Bundle/CoreBundle/Console
/Application.php on line 44

Exception trace:
() at /Applications/MAMP/htdocs/core.git/src/vendor/sensio/distribution-bundle/Composer/ScriptHandler.php:293
Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::executeCommand() at /Applications/MAMP/htdocs/core.git/src/vendor/sensio/distribution-bundle/Composer/ScriptHandler.php:143
Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache() at phar:///usr/local/bin/composer/src/Composer/EventDispatcher/EventDispatcher.php:280
Composer\EventDispatcher\EventDispatcher->executeEventPhpScript() at phar:///usr/local/bin/composer/src/Composer/EventDispatcher/EventDispatcher.php:205
Composer\EventDispatcher\EventDispatcher->doDispatch() at phar:///usr/local/bin/composer/src/Composer/EventDispatcher/EventDispatcher.php:96
Composer\EventDispatcher\EventDispatcher->dispatchScript() at phar:///usr/local/bin/composer/src/Composer/Installer.php:307
Composer\Installer->run() at phar:///usr/local/bin/composer/src/Composer/Command/InstallCommand.php:119
Composer\Command\InstallCommand->execute() at phar:///usr/local/bin/composer/vendor/symfony/console/Command/Command.php:267
Symfony\Component\Console\Command\Command->run() at phar:///usr/local/bin/composer/vendor/symfony/console/Application.php:846
Symfony\Component\Console\Application->doRunCommand() at phar:///usr/local/bin/composer/vendor/symfony/console/Application.php:191
Symfony\Component\Console\Application->doRun() at phar:///usr/local/bin/composer/src/Composer/Console/Application.php:227
Composer\Console\Application->doRun() at phar:///usr/local/bin/composer/vendor/symfony/console/Application.php:122
Symfony\Component\Console\Application->run() at phar:///usr/local/bin/composer/src/Composer/Console/Application.php:100
Composer\Console\Application->run() at phar:///usr/local/bin/composer/bin/composer:54
require() at /usr/local/bin/composer:24

if I downgrade composer requirement for symfony/symfony to 2.8.20 then all returns to normal.

I have installed:

        "name": "sensio/distribution-bundle",
        "version": "v5.0.20",