CAUSE: "Beginning in Android 6.0 (API level 23), users grant permissions to apps while the app is running, not when they install the app." In this case, "ACCESS_FINE_LOCATION" is a "dangerous permission and for that reason, you get this 'java.lang.SecurityException: "gps" location provider requires ACCESS_FINE_LOCATION permission.' error (https://developer.android.com/training/permissions/requesting.html).
SOLUTION: Implementing the code provided at https://developer.android.com/training/permissions/requesting.html under the "Request the permissions you need" and "Handle the permissions request response" headings.