System.NullReferenceException: Object reference not set to an instance of an object.

Hi, I am having problems with this code:

using Android.App;
using Android.Widget;
using Android.OS;

namespace ApplicazioneMatematica
{
[Activity(Label = "ApplicazioneMatematica", MainLauncher = true, Icon = "@drawable/icon")]
public class MainActivity : Activity
{
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);

        // Set our view from the "main" layout resource
        SetContentView (Resource.Layout.Main);

        Button ButtonLayoutEquazioni = FindViewById<Button>(Resource.Id.ButtonLayoutEquazioni);
        Button ButtonLayoutVettori = FindViewById<Button>(Resource.Id.ButtonLayoutVettori);

        ButtonLayoutEquazioni.Click += delegate
        {
            StartActivity(typeof(ActivityEquazioni));
        };

        ButtonLayoutVettori.Click += delegate
        {
            StartActivity(typeof(ActivityVettori));
        };
    }
}

}

When it comes to

        ButtonLayoutEquazioni.Click += delegate
        {
            StartActivity(typeof(ActivityEquazioni));
        };

it says System.NullReferenceException: Object reference not set to an instance of an object. What is going wrong?
It displayed this error only after i modified the activity "ActivityVettori", so it is not even related...
Can you help me?