Redux store returns undefined or null

I just noticed you are setting up the store to null in the source code, whereas in the http://makeitopen.com/docs/en/1-3-data.html tutorials you assign it to a different variable.

The source code:

On make it open site: http://makeitopen.com/docs/en/1-3-data.html

Is there a reason for this? What is the difference between the two? The redux store returns undefined and in some situations returns null. What could be the problem here?