I am not able to reproduce.
try uninstalling Data Assist, then installing it again.
also, reset the workspace layout:
go to Window -> Work Space layout -> Reset '<your layout name>'
and Delete the Dreamweaver Cache file from the users configuration folder:
and before starting the Data Assist wizard, go to Edit -> WebAssist Preferences. in the preview section, disable to preview, and then close the Files and Server Behaviors panels.