Grails 2.5.4 Controller Action Arguments

After upgrading to Grails 2.5.4, one of our projects failed with an exception in the 'canonicalization' phase of the build, due to a NullPointerException in Grails' ControllerActionTransformer:

Caused by: java.lang.NullPointerException
    at org.codehaus.groovy.grails.compiler.web.ControllerActionTransformer.getCodeToInitializeCommandObjects(ControllerActionTransformer.java:424)
    at org.codehaus.groovy.grails.compiler.web.ControllerActionTransformer.addMethodToInvokeClosure(ControllerActionTransformer.java:493)
    at org.codehaus.groovy.grails.compiler.web.ControllerActionTransformer.processClosures(ControllerActionTransformer.java:479)
    at org.codehaus.groovy.grails.compiler.web.ControllerActionTransformer.performInjectionOnAnnotatedClass(ControllerActionTransformer.java:206)
    at org.codehaus.groovy.grails.compiler.web.ControllerActionTransformer.performInjection(ControllerActionTransformer.java:197)
    at org.codehaus.groovy.grails.compiler.injection.GrailsAwareInjectionOperation.call(GrailsAwareInjectionOperation.java:154)
    at org.codehaus.groovy.control.CompilationUnit.applyToPrimaryClassNodes(CompilationUnit.java:1055)
    ... 517 more
| Error Fatal error during compilation org.apache.tools.ant.BuildException: BUG! exception in phase 'canonicalization' in source unit '/home/justin/projects/apps/grails-app/controllers/com/pitchstone/apps/controller/LandingController.groovy' unexpected NullpointerException

Turns out that was because one of our controller actions, which used the old-style "closure" action definitions, had an empty argument list, like this:

def someAction = { ->
    forward action: 'anotherAction'
}

I think once upon a time that action had a command object, and when the action was updated to just forward somewhere else, the command object was deleted from the action's argument list, leaving it with no arguments (instead of the usual single implicit argument, it). Like at one time it probably looked like this:

def someAction = { SomeCommand cmd ->
    [cmd: cmd]
}

Previous versions of Grails were fine with closure-style actions that had no arguments, but not Grails 2.5.4. Fortunately, simply removing the errant arrow (->) fixed the exception (and everything else with Grails 2.5.4 has gone smoothly):

def someAction = {
    forward action: 'anotherAction'
}