An unexpected error while testing?

Question

symfony

A strange error got out when the testing process was already almost completed.

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 151/151 [============================] 100%  - Installing symfony/flex (v1.10.0): Extracting archive
Fatal error: Class Symfony\Flex\Flex contains 2 abstract methods and must therefore be declared abstract or implement the remaining methods (Composer\Plugin\PluginInterface::deactivate, Composer\Plugin\PluginInterface::uninstall) in /builds/CenterJoin/mygitproject/vendor/symfony/flex/src/Flex.php on line 55

Moreover, .gitignorevendor/ is specified and it is not in the Gitlab project. How to fix?