I suspect that this is an unusual case but I thought you might be interested in seeing it.
Attached is a presentation that has two slides each containing a grouped set of objects. It causes an uncaught exception simply by loading and saving. If both groups are ungrouped then the uncaught exception will not happen.
Here is the call stack when it crashes:
java.lang.IllegalArgumentException: Null string passed to TextLayout constructor.java.lang.IllegalArgumentException: Null string passed to TextLayout constructor.
at java.awt.font.TextLayout.(Unknown Source)