Android saving data into datatable but when i click for show it gives me error...check plz

package Database.Use;

import Database.Use.R.string;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class use_database extends Activity
{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        final EditText firstname;
        final Context context = this;
        firstname = (EditText)findViewById(R.id.firstname);
        Button save =(Button)findViewById(R.id.Go);
        Button show = (Button)findViewById(R.id.show);
        save.setOnClickListener(new Button.OnClickListener() {
            public void onClick(View arg0)
            {
             try{  
             SQLiteDatabase myDB;
             String TableName = "tb";
             String ShowData;
            
             myDB= openOrCreateDatabase("db",MODE_PRIVATE, null);
             String query1,query2;
            
             query1="create table if not exists "+TableName+
              ""+"(first VARCHAR );";
             myDB.execSQL(query1);
             query2=("INSERT INTO "+ TableName+ " ( first)
             VALUES ('"+firstname.getText().toString()+"');");
             myDB.execSQL(query2);
           
             }
        catch(Exception e) {
            Log.e("Error", "Error", e);
}
             AlertDialog.Builder alertDialogBuilder = new                            AlertDialog.Builder(context);
                    alertDialogBuilder.setTitle("Salvar");
                    alertDialogBuilder
         .setMessage("Todus Informaroes Incerido!")
    .setCancelable(false)
        .setNegativeButton("Ok",new DialogInterface.OnClickListener() {
    public void onClick(DialogInterface dialog,int id) {
    dialog.cancel();
    }});
        AlertDialog alertDialog = alertDialogBuilder.create();
    alertDialog.show();
        firstname.setText("");
        }});
 //........................... button for show data      
        show.setOnClickListener(new Button.OnClickListener() {

            public void onClick(View arg0)
            {
               
            SQLiteDatabase myDB = null;
            String TableName = "tb";
            Cursor c = myDB.rawQuery("SELECT * FROM " + TableName , null);
            int frist = c.getColumnIndex("first");
            try{
            if(c!=null)
            {
            String str = c.getString(frist);
            firstname.setText(str);
            }
            }catch(Exception e) {
            Log.e("Error", "Error", e);}
// Check result.
}});}}