d3.json: “Uncaught TypeError: Cannot read property 'children' of undefined”

Question

I am trying out d3 as a tool for representing biological data.

I am trying to open the following example in my chrome browser so that I can understand how it works

Answer 1

The code is from d3.v3 but it's pointing to d3.v2, which does not use the error call in the d3.json function.