Saving a user without a password gives this exception.
I'm using Hudson 1.179 and also had this problem in 1.177.

java.lang.NullPointerException
hudson.util.Scrambler.scramble(Scrambler.java:31)

hudson.security.HudsonPrivateSecurityRealm$Details.<init>(HudsonPrivateSecurityRealm.java:139)

hudson.security.HudsonPrivateSecurityRealm$1.newInstance(HudsonPrivateSecurityRealm.java:199)

hudson.security.HudsonPrivateSecurityRealm$1.newInstance(HudsonPrivateSecurityRealm.java:181)
hudson.model.User.doConfigSubmit(User.java:332)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:585)
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:103)
org.kohsuke.stapler.Function.bindAndinvoke(Function.java:59)
org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:63)

org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:240)

org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:298)
org.kohsuke.stapler.Stapler.service(Stapler.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)