improvement to get values ​​from 'map' fields

Hi, I do a minor change,

by sample:

class Record{
    String name
    def values = [key1: 'val1', key2: 'val2']
    def moreValues = [ map1: [key3: 'val3'] ]
}

def withProperties = ['name', 'values.key1', 'values.key2', 'moreValues.map1.key3']

def tempHead = 10
new XlsxExporter(file.getAbsolutePath(),'/tmp/precision_recall.xlsx').with {
     add(records, withProperties, tempHead)
}

In this way will works Ok with maps,, 
else we have: java.lang.NullPointerException: Cannot invoke method containsKey() on null object

thank you very much for your great contribution to grails with this plugin. is very easy and elegant  
Best Redgards, 
Orlando Becerra