@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings)
    {
        Funciones Funciones1 = new Funciones();

        Funciones1.MuestraDialogo(CluefichaActivity.this);

        return true;
    }

    return super.onOptionsItemSelected(item);
}

public class Funciones extends Activity

private AlertDialog.Builder ventana;

Activity miActividad;





@Override
protected Dialog onCreateDialog(int id)
{
    switch (id)
    {
        case DIALOGO:
            ventana = new AlertDialog.Builder(miActividad);

            ventana.setIcon(miActividad.getResources().getIdentifier("my_icon", "drawable", miActividad.getPackageName()));
            ventana.setTitle("title");
            ventana.setMessage("message");

            ventana.setPositiveButton("OK", new DialogInterface.OnClickListener()
                {
                    public void onClick(DialogInterface dialog, int boton)
                    {
                            Intent intent = miActividad.getIntent();
                            miActividad.finish();
                            startActivity(intent);
                    }
                });
            }

            ventana.setNegativeButton("CANCEL", new DialogInterface.OnClickListener()
            {
                public void onClick(DialogInterface dialog, int boton)
                {
                }
            });

            return ventana.create();
    }

    return null;
}

public void MuestraDialogo (Activity actividad)
{
    miActividad = actividad;

    showDialog(DIALOGO);
}

Process: com.myproject.projectname, PID: 1841
java.lang.NullPointerException
        at android.app.Activity.startActivityForResult(Activity.java:3511)
        at android.app.Activity.startActivityForResult(Activity.java:3472)
        at android.app.Activity.startActivity(Activity.java:3714)
        at android.app.Activity.startActivity(Activity.java:3682)
        at com.myproject.projectname.Funciones$1.onClick(Funciones.java:130)
        at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:167)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:157)
        at android.app.ActivityThread.main(ActivityThread.java:5356)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
        at dalvik.system.NativeStart.main(Native Method)