Hack circumventing https://sourceforge.net/p/xslthl/bugs/14/

ws/Docbook/Preprocess/make.rules

@@ -63,7 +63,7 @@ set       toc,title' \
 webhelp.base.dir=.
 
 # Required for docbook Xsl 1.0 based stylesheets
-SAXON6CMD=java -cp "${DBOOK_XSL}/extensions/saxon65.jar:${DBOOK_XSL}/tools/lib/saxon.jar:${DBOOK}/lib/xslthl-2.1.3.jar" \
+SAXON6CMD=java -cp "${DBOOK_XSL}/extensions/saxon65.jar:${DBOOK_XSL}/tools/lib/saxon.jar:${DBOOK_XSL_CUSTOM}/lib/xslthl_npe_patch-2.1.3.jar" \
           '-Dxslthl.config=file://${DBOOK_XSL}/highlighting/xslthl-config.xml' \
           com.icl.saxon.StyleSheet