I figured out how to reproduce this - click on 'create screen' without entering anything into the input field. It creates a blank canvas that doesn't accept any elements.
Yep. That's what I did originally. I was on chrome fedora without it working, but now on chrome mac it works. This time though I did change the name instead of leaving it blank.
Just pushed a fix to not allow blank entires. Sorry again about that :(
The same thing is happening to me. I'm on Chrome v21 in Archlinux. I'm getting the following errors:
"not dropping, no container or component found. builder.js:127"
Looking into it...just pushed a few tweaks. It could also be the default menu wasn't clear what you were supposed to drag and where to drop it. We need to fix that.