Here is more information. The call that requires the DefaultHandler is a jar that is embedded inside the OSGI bundle and placed in the bundle-classpath. This had to be done because the jar uses Class.forName("some.class")
FUSE ESB doesn't export the package of org.xml.sax.helpers from the jre.
You need to change the jre.properties file in $ESB_HOME/etc
How about add Import-Pacakage org.xml.sax.helpers in your customer bundle?