Have you initialized the customExecPath variable?
From the Node JS tutorial the variable should be intialized as follows before creating the appInfo variable.
const app = require('electron').remote.app;
const customExecPath = [process.execPath, app.getAppPath()];
Once appInfo is initialized correctly with the customExePath set, when we call the initializeApp API, a custom URI for your application is registered. Once the authenticator authorizes your application it opens the uri which has the authorization string response.
Further down in the tutorial you will see how the uponAuthResponse function receives this auth response and calls the loginFromUri API to establish a connection to the network.