feat(rxNotify): Check the stack exists before clearing it

Might not really be a bug per se.
Nevertheless I've seen myself and the DCX folk be in the situation where we might call clear on a stack that has not been initialized (but will be right after). 

I think the logic of how we clear the stack in our projects is worth looking at also.  Nevertheless, wanted to bring it up as a potential addition. 


I don't think you should be calling the function in this test. Chai/Mocha will run the function in a 'try'/'catch' statement to avoid the tests erroring if there is an issue.
http://chaijs.com/api/bdd/#-throw-constructor-
```
expect(fn).to.throw(ReferenceError);
```