Get invalid XML element instance

I've implemented an XML validator using this approach: How to get the element of and invalid xml file with failed xsd Validation

SAXParseException allows me to get line number, column number etc. What I wan is to record values of the element instance that was invalid. For example, if the XSD is:

<xs:schema xmlns:xs="http://ift.tt/tphNwY" elementFormDefault="qualified"
xmlns:vc="http://ift.tt/REsXlC" vc:minVersion="1.1">
    <xs:element name="dimension">
       <xs:complexType>
          <xs:sequence>
               <xs:element name="height" type="xs:integer"/>
               <xs:element name="width" type="xs:integer"/>
           </xs:sequence>
           <xs:assert test="height le width"/>
       </xs:complexType>
   </xs:element>

I want to record the height or width of the dimension that was invalid. How do I do that?

The article proposes using validator.getProperty. But that returns java.lang.NullPointerException.