Note a strange behavior here. As you show the modal alert when asking for an interface as seen in [line 283](https://github.com/mozilla-b2g/gaia/pull/13708/files#diff-7867a1ad017c0846d7dae5e20c3b85b9R283) in `common.js` then return `undefined` so this statement should be executed, you will show the modal dialog twice. First time, when the user closes the dialog nothing happens; second time, the application is closed. Annoying for the final user.

My recommendations are:
  1. Leave only the check when trying to obtain an interface so you can remove this `else`.
  2. Do not close the application, just avoid the UI to crash. 
  3. Add a better clarification of what is happening. Instead of `Mobile data usage is not available` which is not always true.

About the point number 3, note we added this check because the `loadNetworkInterfaces()` is asynchronous so we can be in one of three states:
  * Not yet loaded: if this happen we should alert with `Interfaces not yet available`.
  * Already loaded: no alert should be shown
  * Failed to load: log an error and show an alert with `Interfaces could not be loaded` or something like that.