JDK classes not detected in IntelliJ in Ubuntu with Open JDK 1.8

I ran into this issue while trying to open a Gradle project in IntelliJ 2018. The solution is to set the JVM again from the settings for Gradle as shown below. If that doesn’t work, close the IDE, blow up the settings directory of IntelliJ with
rm -fr ~/.IntelliJIdea2018.3/
Then reopen the project.

screenshot from 2019-01-05 09-45-20