I've managed to get a pretty simple sample that breaks it.
If you cut and paste this code into the XML Editor (Web Lang Addon) sample in your sample pack it cashes.
Its the comment within the DTD thats causing the problems.
<!-- My Test Branding Line -->
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!-- dfsdf -->
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
<body>Don't forget me this weekend</body>