Griffon, a mutual binding with validation (not working)

griffon-binding-mutual-validator

import groovy.beans.Bindable
import groovy.swing.SwingBuilder

class MyModel {
  @Bindable String value
}

def model = new MyModel()

def isNumber = { val ->
  if(!val) return true
  try {
    println "->isNumber($val): model.value: ${model.value}"
    Double.parseDouble(val)
  } catch(NumberFormatException e) {
    false
  }
}

def swing  = new SwingBuilder()
swing.edt {
  frame(title: 'Binding', pack: true, visible: true) {
    gridLayout(cols: 2, rows: 3)
    label 'Validator'
    
    //OPT1 should work but it doesnt
    textField(columns: 20, text: bind('value', source: model, mutual:true,validator: isNumber))
    //OPT2 validator: ok, mutual: ko (cannot initialize model.value)
    //textField(columns: 20, text: bind('value', target: model, mutual:true,validator: isNumber))    
    
    label 'Only numbers here'
    textField(columns: 20, text:  bind('value', source: model))
  }
}