https://issues.apache.org/jira/browse/CAMEL-10253
Workaround:
Declare exception as bean:
<bean id="illegalArgumentException" class="java.lang.IllegalArgumentException"/>
and then throw exception like:
<throwException message="Unknown operation ${header.operationName} called"
exceptionType="java.lang.IllegalArgumentException" ref="illegalArgumentException"/>
This way both the type and the exception is set so that no nullpointer can happen.

Where are you experiencing the behavior? What environment?

JBoss Fuse 6.3.0 R3 - R8