Help please - Entering StartButton related java code breaks my app.

adding StartButton related code generates error while running on emulator. Error Message displayed is "Unfortunately, Interactive Story has stopped".

I tried following,

1 declaring mStartButton as private shows that - Private field 'mStartButton' is never assigned. so I declared it public and message goes away.

Here is my MainActivity.java code:

package au.com.relteksys.interactivestory;

import android.app.Activity; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.EditText; import android.widget.Toast;

public class MainActivity extends Activity {

private EditText mNameField; private Button mStartButton;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mNameField = (EditText)findViewById(R.id.nameEditText);

    mStartButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String name = mNameField.getText().toString();
            Toast.makeText(MainActivity.this, name,    Toast.LENGTH_LONG).show();
        }
    });
}

}