Run the following commands:

cd core-plugin-openbis
JAVA_HOME=<path to home of jdk 8> ./gradlew buildCoreUIPackageUsingJDK8

Example for <path to home of jdk 8>: /Library/Java/JavaVirtualMachines/adoptopenjdk-8.jdk/Contents/Home/

A new version of resource/gwt.tar.gz has been created and should committed to the git repository.