Null Pointer Exception error in SQlite insertion of data

I am a beginner in android apps and was trying to insert data using SQlite...But I get a null pointer exception when trying to access the line " SQLiteDatabase db = this.getWritableDatabase();".I checked the similar questions in this site but was not able to understand the error Please Help...Thanks

Here is my DatabaseOperations.java class

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
import android.widget.Toast;

import com.example.admin.locationapp.DataTable.TableInfo;

public class DataOperations extends SQLiteOpenHelper {
public static final int database_version=1;
public String CREATE_QUERY="CREATE TABLE "+TableInfo.TABLE_NAME+"("+TableInfo.STORE_NAME+" TEXT,"+TableInfo.lATITUDE+" TEXT,"+TableInfo.lONGITUDE+" TEXT,"+TableInfo.PHONE+" INTEGER);";

public DataOperations(Context context) {
    super(context, TableInfo.DATABASE_NAME, null, database_version);
    Log.d("Database Operations","Database Created");
}


@Override
public void onCreate(SQLiteDatabase db) {

    db.execSQL(CREATE_QUERY);

    Log.d("Database Operations", "Tables Created");

}


    public void putInformation(DataOperations dob,String Name,String Latitude,String Longitude) {
        try {

            SQLiteDatabase SQ = dob.getWritableDatabase();  //error here
            ContentValues cv = new ContentValues();
            cv.put(TableInfo.STORE_NAME, Name);
            cv.put(TableInfo.lATITUDE, Latitude);
            cv.put(TableInfo.lONGITUDE, Longitude);
            //  cv.put(TableInfo.PHONE, Bal);


            SQ.insert(TableInfo.TABLE_NAME, null, cv);

            Log.d("Database Operations", "One  income Row inserted");
        }catch(Exception e)
        {
            e.printStackTrace();
        }
    }



@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS"+ TableInfo.TABLE_NAME);

Log.d("Database Operations", "Table Upgraded");

}


}

Here is DataTable.java class

import android.provider.BaseColumns;

public class DataTable {

public static abstract class TableInfo implements BaseColumns
{public static final String TAG1="DBHelper";

//columns for income
public static final String STORE_NAME="store";
public static final String lATITUDE="latitude";
public static final String lONGITUDE="longitude";
public static final String PHONE="phone";


public static final String TABLE_NAME="offlinemode";
public static final String TABLE_NAME_TAG="TAG";


public static final String TAG="Tag";


public static final String DATABASE_NAME="locationApp";
public static final int DATABASE_VERSION=1;
}



}

MainActivity.class

public class MainActivity extends AppCompatActivity{


Context ctx=this;
public static String[] nameArray;
public static String[] distArray;
public static String[] latArray;
public static String[] lonArray;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    try {

       DataOperations DB=new DataOperations(ctx);

            String name=MainActivity.nameArray[0].toString();
            String latitude=MainActivity.latArray[0].toString();
            String longitude=MainActivity.lonArray[0].toString();

            DB.putInformation(DB, name, latitude, longitude);

    } catch (Exception e) {
e.printStackTrace();
    }
}

}

And the logcat

W/System.err: java.lang.NullPointerException
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:256)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at com.example.admin.locationapp.DataOperations.putInformation(DataOperations.java:48)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at com.example.admin.locationapp.MainActivity.printRes(MainActivity.java:266)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at com.example.admin.locationapp.AsyncCaller.onPostExecute(AsyncCaller.java:91)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at com.example.admin.locationapp.AsyncCaller.onPostExecute(AsyncCaller.java:14)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.os.AsyncTask.finish(AsyncTask.java:632)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.os.AsyncTask.access$600(AsyncTask.java:177)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:102)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.os.Looper.loop(Looper.java:136)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:5021)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at java.lang.reflect.Method.invokeNative(Native Method)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at java.lang.reflect.Method.invoke(Method.java:515)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:827)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:643)
01-12 12:40:32.562 30514-30514/com.example.admin.locationapp W/System.err:     at dalvik.system.NativeStart.main(Native Method)