Update Ractive.js

I my app sometimes I get "Uncaught TypeError: Cannot call method 'teardown' of null" JavaScript error.
I'm not sure why is it happening. Probably it tries to teardown some unexisting node?
The above code eliminates the error, I don't know if it fixes the problem. 
I don't know how Ractive internals work but maybe it will help you to figure out what causing the bug. Tell me if you need more info.