Unhandled Exception: Java.Lang.ClassNotFoundException ??

Question

hello , i write this code for my android application

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

namespace myfitnessapp
{
    [Activity(Label = "myfitnessapp", MainLauncher = true)]
    public class MainActivity : Activity
    {
        private Button UpperbodyButton;
        private Button LowerbodyButton;
        private Button CardioButton;
        private Button YogaButton;

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

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

        }
        private void FindViews()
        {
            UpperbodyButton = FindViewById<Button>(Resource.Id.UpperbodyButton);
            LowerbodyButton = FindViewById<Button>(Resource.Id.LowerbodyButton);
            CardioButton = FindViewById<Button>(Resource.Id.CardioButton);
            YogaButton = FindViewById<Button>(Resource.Id.YogaButton);
        }
        private void HandleEvents()
        {
            UpperbodyButton.Click += UpperbodyButton_Click;
            LowerbodyButton.Click += LowerbodyButton_Click;
        }

        private void UpperbodyButton_Click(object sender, System.EventArgs e)
        {
            var intent = new Intent(this, typeof(Upperbody2));
            StartActivity(intent);
        }

        private void LowerbodyButton_Click(object sender, System.EventArgs e)
        {
            var intent = new Intent(this, typeof(Lowerbody2));
            StartActivity(intent);
        }
    }
}

but when i tried to run it , it show me this error : 

Unhandled Exception:

Java.Lang.ClassNotFoundException: md56c2df86cd51c12517412b6ac46cbff79.Lowerbody2

 

just for the second button , i don't know how to slve it .