App crashes when the user cancels Dropbox authentication

[MarquisLP]

Environment

• App version: 1.7.0
• Android version: Any
• Devices affected: Any

Steps to Reproduce

• In ArticleListActivity, open the overflow menu and select 'Backup to Dropbox'
• Once the Dropbox pop-up finishes loading, press the Android 'Back' button to close the pop-up

Expected Result: the app returns to the ArticleListActivity.

Actual Result: the app crashes and the user is brought to the device's home screen without any error message.

Notes

The fix is probably to add a null check in ArticleListActivity.storeDropboxCredentials(), as Dropbox.Auth.getDbxCredential() will return a null credentials object if the user did not authenticate.