Most likely this is a memory issue with DW.
Dreamweaver uses a lot of Memory when Drawing UI pages and is not very good about freeing memory it no longer needs.
I have run into a similar situation a couple of times and have had to reboot the computer and make sure any unneeded programs are not running while working with Dreamweaver.
Another option is to add one page, then save a Preset, start the wizard again and add the second page, then save the preset. Continue on builing each page and saving the preset until the preset is complete.
We will be adding an auto recover feature in a future update so that you will be able to recover your progress when DW Crashes.