I just had the same problem. Solved it by installing openjdk-jre-8, then running
update-alternatives --config java
which gave me this (yours may be different):
There are 3 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/lib/jvm/java-13-openjdk-amd64/bin/java 1211 auto mode
1 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1111 manual mode
2 /usr/lib/jvm/java-13-openjdk-amd64/bin/java 1211 manual mode
3 /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java 1081 manual mode
so now I could cd into my WhiteboxGAT dir and run
/usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java -jar WhiteboxGIS.jar
and it worked fine.