Forge wraps exceptions. And during that, it 1) changes the name, 2) hides the message.

Example:
org.jboss.windup.util.exception.WindupException_$$_javassist_15b124f6-0ec8-431e-b7f9-cd561373c32c: null
at org.jboss.windup.rules.apps.java.scan.operation.UnzipArchiveToTemporaryFolder.perform(UnzipArchiveToTemporaryFolder.java:46)
at org.jboss.windup.rules.apps.java.scan.operation.UnzipArchiveToTemporaryFolder.perform(UnzipArchiveToTemporaryFolder.java:23)

When you look at the line throwing the exception, it's clear that the message can't be null.

STR:
1) Checkout 408fca
2) `mvn clean install`.