SQLite not inserting or returning any rows

My Android application correctly creates a database with a single table, but when a method is called to insert another row into the table, it fails and returns no error.

My goal is to have the method addOffice() add a row to my SQLite DB table Offices, then check to see if any rows exist, using the method ifExists().

(For ease-of-reading, I've only included relevant code.)

MainActivity.java:

public class MainActivity extends AppCompatActivity {

private static String TAG = MainActivity.class.getSimpleName();
TextView testText = null;
DBHelper dbHelper = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    testText = (TextView) findViewById(R.id.testText);
    dbHelper = new DBHelper(this, null, null, 1);

    // Creates a new Office object, setting items _id, name, and acro.

    Office office = new Office(1, "testOffice", "TEST");
    dbHelper.addOffice(office);

    testText.setText(String.valueOf(dbHelper.ifExists("Offices")));

}

DBHelper.java:

public class DBHelper extends SQLiteOpenHelper{

public static final int DATABASE_VERSION = 1;
public static final String TAG = SQLiteOpenHelper.class.getSimpleName();
public static final String DATABASE_NAME = "emergency.db";

public DBHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
    super(context, DATABASE_NAME, factory, DATABASE_VERSION);
}

@Override
public void onCreate(SQLiteDatabase db) {
    String query =  "CREATE TABLE Contacts( " +
                    "_id INTEGER PRIMARY_KEY, " +
                    "office_id INTEGER, " +
                    "name TEXT," +
                    "phone TEXT);";
    db.execSQL(query);

    query = "CREATE TABLE Offices( " +
            "_id INTEGER PRIMARY_KEY, " +
            "acro TEXT, " +
            "name TEXT);";
    db.execSQL(query);
    db.close();
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    db.execSQL("DROP TABLE IF EXISTS Contacts;");
    db.execSQL("DROP TABLE IF EXISTS Offices;");
    onCreate(db);
}



public void addOffice (Office office){
    ContentValues values = new ContentValues();
    values.put("_id", office.get_id());
    values.put("acro", office.get_acro());
    values.put("name", office.get_name());

    SQLiteDatabase db = getWritableDatabase();

    db.insert("Offices", null, values);
    db.close();

}


public boolean ifExists (String tableName) {
    SQLiteDatabase db = getReadableDatabase();
    String query = "SELECT * FROM " + tableName;
    Cursor c = db.rawQuery(query, null);
    if(c.getCount() <= 0){
        c.close();
        return false;
    }
    c.close();
    return true;
}

public String dbToString() {
    String result = "default";
    SQLiteDatabase db = getWritableDatabase();
    String query = "SELECT * FROM Offices";
    Cursor c = db.rawQuery(query, null);
    c.moveToFirst();
    while (!c.isAfterLast()) {
        if (c.getString(c.getColumnIndex("_id")) != null) {
            result += c.getString((c.getColumnIndex("_id"))) + "\n";
        } else {
            result += "_id was NULL";
        }
    }
    c.close();
    db.close();
    return result;
}
}

So far, ifExists() has been consistently returning false to testText, and when directly returning data from the table using dbToString(), only "default" is returned.

I know that this should be relatively simple, but I've been stuck on this for longer than I should be. Any help is appreciated.