Network error when attempting to add products to resource list

[JApp]

Hi, I'm trying to build my fist app using the steps located at; https://shopify.dev/apps/getting-started/add-functionality But I'm stuck on the fourth step. Whenever I click on the Add button to add the selected products to the resource list, I keep getting the following error message -  Network error: Unexpected token I in JSON at position 0.

I would really appreciate it if someone could help me with sorting this out.