when I click button 4 I get a big run time error

This is a small android code to implement an exammple of actionlistener

To test this plz install this in your phone.

then first click button 4

then click button 10 and you will get the error.

This program has been written in adt.

enter code here

package com.example.test;
import java.util.Random;
import android.os.Bundle;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.graphics.Color;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends Activity implements OnClickListener {
Button b[];

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        b=new Button[11];    //declared an array of buttons and defined each button. 

        b[0] = (Button) findViewById(R.id.btn1);
        b[1] = (Button) findViewById(R.id.btn2);
        b[2] = (Button) findViewById(R.id.btn3);
        b[3] = (Button) findViewById(R.id.btn4);
        b[4] = (Button) findViewById(R.id.btn5);
        b[5] = (Button) findViewById(R.id.btn6);
        b[6] = (Button) findViewById(R.id.btn7);
        b[7] = (Button) findViewById(R.id.btn8);
        b[8] = (Button) findViewById(R.id.btn9);
        b[9] = (Button) findViewById(R.id.btn10);
        b[10] = (Button) findViewById(R.id.btn11);

        for(int i=0;i<10;i++)
        {
            b[i].setOnClickListener(this);//set onClickListener on all buttons (here is where I think the execution is going wrong)
        }

      }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }


    @SuppressLint("ShowToast") @Override
    public void onClick(View v){

        if(v.getId() == R.id.btn10)
        {
            Toast.makeText(getApplicationContext(), "Click a button to change its color", 3).show();   //displaying a message
            if(v.getId() == R.id.btn1) //checks which button is clicked
            {
                b[0].setBackgroundColor(Color.GREEN); //on clicking this command will get executed
            }
            else if(v.getId() == R.id.btn2)
            {
                b[1].setBackgroundColor(Color.GREEN);
            }
            else if(v.getId() == R.id.btn3)
            {
                b[2].setBackgroundColor(Color.GREEN);
            }
            else if(v.getId() == R.id.btn4)
            {
                b[3].setBackgroundColor(Color.GREEN);
            }
            else if(v.getId() == R.id.btn5)
            {
                b[4].setBackgroundColor(Color.GREEN);
            }
            else if(v.getId() == R.id.btn6)
            {
                b[5].setBackgroundColor(Color.GREEN);
            }
            else if(v.getId() == R.id.btn7)
            {
                b[6].setBackgroundColor(Color.GREEN);
            }
            else if(v.getId() == R.id.btn8)
            {
                b[7].setBackgroundColor(Color.GREEN);
            }
            else if(v.getId() == R.id.btn9)
            {
                b[8].setBackgroundColor(Color.GREEN);
            }
            else
            {
                Toast.makeText(getApplicationContext(), "You clicked a wrong button", 3).show();
            }
        }
        else if(v.getId() == R.id.btn11)
        {
            Toast.makeText(getApplicationContext(), "Randomly a button will change color", 3).show();
            Random r = new Random();

            for(int i = 0 ; i < 10 ; i++)
            {
                int i1 = r.nextInt(9);
                b[i1].setBackgroundColor(Color.GREEN);
            }

        }
        else
        {
            Toast.makeText(getApplicationContext(), "Select only btn10 and btn11", 3).show();
        }
    }
}