This is a frequent issue with data assist
A quick look around the web and you will find dozens of support requests regarding this issue.
We have narrowed it down to the fact that Data Assist attempts to store all the variables to build the initial pages, then it attempts to build a preview. If you have a fairly large table that you are working with this may go beyond the limit of your cache and Webassist crashes Dreamweaver.
It would be great if we could simply disable the "preview" windows. These pre-set themes are low grade in the first place and no modern day design would actually use the icons, nor layouts.
I have yet to find a definitive answer - only ones similar to this thread -
"Not sure if that is what fixed it...." WA - "Okay glad to hear its working" - No finalized answer!