I have found the cause of this problem! The problem seems to be with my pointer. I don't know why it wasn't crashing in Flash, too, but there you go. Flash just gives a null object reference warning, as shown below.
But a problem remains with this -- I can't get into Data Structures to fix the pointer issue. This problem is addressed in a different post, and is being experienced by multiple people. Anytime I click on anything in the data structures extension I get an error message and can't go any further. These appear in my log when I run it in Flash. I've tried re-installing to no effect. I'm now using the latest build of Stencyl.