android - setOnItemClickListener is not working

Question

I try using setOnItemClickListener on my GridView with some Buttons but it doesn't work. This is my CustomGrid adapter:

package com.example.omar.soundboard;
/**
 * Created by Omar on 10/02/2015.
 */
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.RelativeLayout;


import java.util.ArrayList;

public class CustomGrid extends BaseAdapter
{
    private Context mContext;
    private ArrayList<String> arr;

    public CustomGrid(Context c, ArrayList<String> arr)
    {
        mContext = c;
        this.arr = arr;
    }

    @Override
    public int getCount()
    {
        return arr.size();
    }

    @Override
    public Object getItem(int position)
    {
        return arr.get(position);
    }

    @Override
    public long getItemId(int position)
    {
        return position;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent)
    {
        View grid;
        LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        if (convertView == null)
        {
            grid = new View(mContext);
            grid = inflater.inflate(R.layout.grid_single, null);
            Button btn = (Button) grid.findViewById(R.id.button);
            btn.setLayoutParams(new RelativeLayout.LayoutParams(100,75));
            btn.setPadding(1,1,1,1);
            btn.setText(arr.get(position));
        }
        else
        {
            grid = (View) convertView;
        }
        return grid;
    }
}

This is the grid_single.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" >

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Prova"
        android:id="@+id/button"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />
</RelativeLayout>

This is the activity code:

package com.example.omar.soundboard;

    import android.support.v7.app.ActionBarActivity;
    import android.os.Bundle;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.View;
    import android.widget.AdapterView;
    import android.widget.AdapterView.OnItemClickListener;
    import android.widget.Button;
    import android.widget.GridView;
    import android.widget.Toast;

    import java.util.ArrayList;


    public class MainActivity extends ActionBarActivity {

        private GridView grid;
        private ArrayList<String> nomi;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            nomi = new ArrayList<String>();
            nomi.add("Cosimo");
            nomi.add("Angela");
            nomi.add("Prova");
            nomi.add("Prova2");
            final CustomGrid adapter = new CustomGrid(this, nomi);
            grid = (GridView)findViewById(R.id.gridSound);
            grid.setAdapter(adapter);
            grid.setOnItemClickListener(new OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                    Toast.makeText(MainActivity.this, "Test", Toast.LENGTH_LONG).show();
                }
            });
        }

This is the activity_main.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="0dp"
    android:paddingRight="0dp"
    android:paddingTop="0dp"
    android:paddingBottom="0dp" tools:context=".MainActivity">

    <GridView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/gridSound"
        android:numColumns="3"
        android:layout_alignParentTop="true"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />
</RelativeLayout>

I'm testing it on Android 2.3.3, the buttons are displayed but when I press on one of them nothing happens! Can someone help me? I can't figure out what's the problem.