Crash with roMessageDialog

I found that if you call UpdateText() on a roMessageDialog before SetText() has been called, this will crash the Roku box. I guess this is not terribly unexpected, but it probably should just be a BrightScript crash, rather than take the whole box down.

Also, there isn't any documentation for SetText() on the roMessageDialog page: http://sdkdocs.roku.com/display/sdkdoc/ifMessageDialog