I __think__ I figured it out. Silly me, I didn't scope out the preferences before using the tool.
What may have happened is I was looking at a SWF, exported it, etc.... then I closed the decompiler. Then I moved the project to a different folder.
Unbeknown st to me, there's a default setting in "General Configuration" which has "Open last browsing file" as the default for Startup. Since my previously-viewed file was moved the program was aborting.
That's my best guess so far. I changed this setting, and the program seems to startup cleanly every time.
Now I can see what this tool can do.... hopefully it lives up to the hype