I'd like to fix this issue, but I'm having a hard time reproducing it. When I switch to the text only preset it automatically selects the first item in the tree so that the error doesn't occur.
Are there any other steps to reproduction, or just open a preset and change the design type? Maybe it is my version of Dreamweaver? What version of Dreamweaver are you using? I need to figure out how to reproduce this so I can get it fixed.