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For the network requests to work you must set your own API_KEY value in your gradle.properties file. Example:
API_KEY="YOUR_API_KEY_HERE"
/gradle.properties is listed in the .gitignore file so you can safely add this file locally.
If you do not have an api key, first register with themoviedb.org and then request API access from https://www.themoviedb.org/settings/api