Thanks for posting.
Does it only crash when you attempt to change a field name or can you reproduce it with other actions as well? What version of Bento are you using? You can get this info through Bento -> About Bento.
Let's try removing the plist files. Go to /Users/<your user name>/Library/Preferences/ and move any files that begin with com.filemaker to the trash. After that, attempt the import again and let us know the result.