DatabaseException: Firebase crashes only on signed APK

Question

My app works fine on the simulator and on my physical device when I run via Android Studio, but it immediately crashes when I use a signed APK (or install it from Play Store). I used Android Studio's Firebase Assistant to setup everything.

Here's my onCreate:

FirebaseDatabase database = FirebaseDatabase.getInstance();
        database.setPersistenceEnabled(true);
        DatabaseReference myRef = database.getReference();

        final Activity context = this;
        // Read from the database
        myRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                GenericTypeIndicator<List<University>> genericTypeIndicator = new GenericTypeIndicator<List<University>>() {};
                colleges = dataSnapshot.getValue(genericTypeIndicator);

                List<String> names = new ArrayList<String>();

                for (University college : colleges){
                    names.add(college.name);
                }

                if (colleges.size() > 0) {
                    CustomList customList = new CustomList(context, colleges, names);
                    listView.setAdapter(customList);
                    setListeners();
                }

            }

            @Override
            public void onCancelled(DatabaseError error) {
                // Failed to read value
                Log.w(TAG, "Failed to read value.", error.toException());
            }
        });