The following method of the class org.apache.xml.security.transforms.Transform does not test if the contextChild parameter is null:
public static final Transform getInstance(
    Document doc, 
    String algorithmURI, 
    Element contextChild)

I propose this modified code to fix this bug:
public static final Transform getInstance(
    Document doc, String algorithmURI, Element contextChild)
    throws InvalidTransformException {

    HelperNodeList contextNodes = null;

    if (contextChild != null) {
        contextNodes = new HelperNodeList();

        XMLUtils.addReturnToElement(doc, contextNodes);
        contextNodes.appendChild(contextChild);
        XMLUtils.addReturnToElement(doc, contextNodes);
    }

    return Transform.getInstance(doc, algorithmURI, contextNodes);
}