I'm not able to get this error to occur for me in a very similar setup to what you have described. The problem might be due to a corruption of your configuration folder or it may be related to the preview. The first thing to try would be to disable the preview. To do this go to Edit > WebAssist Preferences then click on preview and check the box to disable it. This will ensure that the preview is disabled. Give this a try to see if you get past the problem.
If you do get past the problem then it points to the preview as the issue. To resolve the issue you may need to rename or delete your configuration folder. Here is a link with steps you can take to do that:
Please post back and let us know how this works for you.