Thanks for your response, and my apologies for the trouble you're having.
I've tested and confirmed this issue, and I've also confirmed that in our latest development builds this issue no longer occurs - there's no crash anymore.
Architect also adds an Ajax Proxy by default since it's a common behavior. If you're just pasting in local JSON for your root (you may not be, I just don't have more context on your real world use case), be sure to delete that proxy before putting in your root for the time being.
When Architect 2.2 is released, however, the crash issue will be alleviated.
Apologies once again for the trouble, and thank you for providing us a simple way of replicating the issue - it helps us make Architect better!
Sencha Architect Development Team