I've seen people post errors like this. The times I've seen are when the designer appears to try to show an animation. For example I've seen it occur when the designer window is resized or maximized (though this does not seem repeatable).
To me it seems to be related to an animation because when when I see the error message the designer background is grayed out. If I press 'Ignore' the next step in the animation occurs and the background lightens. After pressing the 'Ignore' button several times the animation completes, the designer window is not greyed out and the error message no longer appears.
Thanks for the insight. You are probably right about an animation triggering this specific issue.
In our production builds, we do not currently have a way to denote between one bug and another. The error which you have seen displayed could be the result of many different bugs. We will be fixing this issue so that the error messages our customers report actually have meaning for us.
For right now, we need the steps required to reproduce the issue and then we will get a "real" error message that tells us what the root of the problem is instead of just the generic "TypeError: 'undefined' is not an object".
BTW, I've become more impressed the more I've used the designer and realized how its features work and can be used to do most things plus, of course, no guessing about the parameters names and options.
I'm looking forward to future iterations as you build out the functionality.