Mozilla Add-on: Google Translator returns null

Mozilla Add-on: Google Translator returns null

I just started to build Mozilla add-ons, and I am trying to run the
example (Google Translate & replace the selected text) shown in the
official introduction video. But it is failing with "TypeError:
response.json.responseData is null". Can anyone help me to fix the
"Translator" code. I tried the code in both Add-on SDK and Add-on Builder.
Link to the tutorial video:
https://addons.mozilla.org/en-US/developers/builder
Regards, Mohan