Lesson Builder doesn't handle null short answers

We have a user generating an IllegalArgumentException based on what looks like a null 'Short Answer' response. The user just has to visit the page to generate the following error. I've attached a diff that should resolve the problem.

• • • traceback is from branch at https://source.sakaiproject.org/svn/msub/umich.edu/ctools/PATCHED/lessonbuilder/branches/CTools-2.9_from-2.9.3

Fatal internal error handling request:
Target exception of class java.lang.IllegalArgumentException
Successive lines until stack trace show causes progressing to exception site:
Value of UIBoundString cannot be null
java.lang.IllegalArgumentException: Value of UIBoundString cannot be null
at uk.org.ponder.rsf.components.UIBoundString.setValue(UIBoundString.java:12)
at org.sakaiproject.lessonbuildertool.tool.producers.ShowPageProducer.fillComponents(ShowPageProducer.java:2189)