Maybe try upping your heap/permgen? It's an odd error to be seeing for sure, but since it's happening when all the javascript is being minified maybe there's an OOM happening somewhere that's being hidden by that NPE. I have this in my .bashrc:
MAVEN_OPTS="-Xmx2048m -XX:PermSize=256m -XX:MaxPermSize=512m"; export MAVEN_OPTS
haven't seen a problem, and I even did a test build right now just for you :-)