/**
* According to FXG 2.0 spec., <mask> must be before any graphical element.
*/
if (children != null)
{
problems.add(new FXGInvalidMaskElementProblem(getDocumentPath(), child.getStartLine(), child.getStartColumn()));
return;
}
if (mask == null)
{
mask = ((MaskPropertyNode)child).mask;