Groovy Puzzler: null + null is?

Does the following code run without an exception and what does the following code print out?

def isNull = null
isNull += null
println isNull.getClass()

GORM Gotchas: Validation and hasMany

The excellent GORM Gotchas Series inspired me to write about a gotcha I found recently.
You have a domain class Container which contains elements:

class Container {
  static hasMany = [elements:Element]

  List<Element> elements
}

and the element has a constraint:

class Element {
  static belongsTo = [container:Container]

  static constraints = {
    email(email: true)
  }

  String email
}

When you try to save a container with more than one element that fails validation, only the first error appears:

Container c = new Container()
c.addToElements(new Element(email: "a"))
c.addToElements(new Element(email: "b"))
c.save()
assertEquals(2, c.errors.allErrors.size()) // fails, only one error is recorded!

The solution described in the docs coined with In some situations (unusual situations)) is to use a custom validator in the container class:

class Container {

  static constraints = {
      elements(validator: { List val, Container obj ->
          val.each {Element element ->
            if(! element.validate()) {
              element.errors.allErrors.each { error->
                obj.errors.rejectValue(
                      'elements',
                      error.getCode(),
                      error.getArguments(),
                      error.getDefaultMessage()
                )
              }
            }
          }
          return true
      })
  }

  static hasMany = [elements:Element]

  List<Element> elements
}