Call to undefined method PhpParser\Node\UnionType::toString() in Comparator/Type.php with php7.3

[vukanac]

The smallest number of errors I get with php7.3 but still has one:

$ composer create-project tomzx/php-semver-checker-git --stability=dev ../semver
$ docker run -it --rm -v $(PWD)/../semver:/semver -v $(PWD):/app -w /app php:7.3-cli-alpine \
       sh -c "apk add git && php /semver/bin/php-semver-checker-git suggest -vvv --include-before=src --include-after=src --details --allow-detached"

Fatal error: Uncaught Error: Call to undefined method PhpParser\Node\UnionType::toString()
in /semver/vendor/tomzx/php-semver-checker/src/PHPSemVerChecker/Comparator/Type.php:35