System.reflection.targetinvocationexception in xamarin forms

System.Reflection.TargetInvocationException: 'Exception has been thrown by the target of an invocation.'

public class MTNRCHelper
   {
       public readonly SQLiteAsyncConnection db;
       public MTNRCHelper(string localDb)
       {
           db = new SQLiteAsyncConnection(localDb);
           db.CreateTableAsync<Materials>();

         // db.DropTableAsync<Materials>();

           db.CreateTableAsync<ToolsDetailNewList>();
          // db.DropTableAsync<ToolsDetailNewList>();


       }

       public async Task<int> SaveMaterial(Materials obj)
        {
        return await db.InsertAsync(obj);
        }
       public async Task<List<Materials>> GetMaterialList()
       {
           return await db.Table<Materials>().ToListAsync();
        }
       public async Task<int> UpdateMaterial(Materials obj)
       {
           return await db.UpdateAsync(obj);
       }
       public async Task<int> DeleteMaterial(Materials obj)
       {
           return await db.DeleteAsync(obj);
       }

       //Method For Tools
       public async Task<int> SaveTools(ToolsDetailNewList obj)
       {
           return await db.InsertAsync(obj);


       }
       public async Task<List<ToolsDetailNewList>> GetToolsList()
       {

           return await db.Table<ToolsDetailNewList>().ToListAsync();

       }
       public async Task<int> UpdateTools(ToolsDetailNewList obj)
       {
           return await db.UpdateAsync(obj);
       }
       public async Task<int> DeleteTools(ToolsDetailNewList obj)
       {
           return await db.DeleteAsync(obj);
       }



   }

private static  MTNRCHelper db;
		public static  MTNRCHelper MyDababase
		{
			get
			{
				if (db == null)
				{
					db = new MTNRCHelper(Path.Combine(System.Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), "LocalDb.db3"));
				}
				return db;
			
			}
		}

private void btnsave_Clicked(object sender, EventArgs e)
{


        if (string.IsNullOrEmpty(txtNPD_ALTERNATE_OFFPART_NO.Text) ||
            string.IsNullOrWhiteSpace(txtNPD_ALTERNATE_OFFPART_NO.Text)||
            string.IsNullOrEmpty(txtOffSerialNumber.Text) ||
            string.IsNullOrWhiteSpace(txtOffSerialNumber.Text) ||
            string.IsNullOrEmpty(txtonPart.Text) ||
            string.IsNullOrWhiteSpace(txtonPart.Text) ||
            string.IsNullOrEmpty(txtOnPArtNumbers.Text) ||
            string.IsNullOrWhiteSpace(txtOnPArtNumbers.Text) ||
            string.IsNullOrEmpty(txtPartType.Text) ||
            string.IsNullOrWhiteSpace(txtPartType.Text) ||
            string.IsNullOrEmpty(txtNPD_ALTERNATE_PART_NO.Text) ||
             string.IsNullOrWhiteSpace(txtNPD_ALTERNATE_PART_NO.Text) ||
            string.IsNullOrEmpty(txtNPD_OFF_S_NO.Text) ||
             string.IsNullOrWhiteSpace(txtNPD_OFF_S_NO.Text) ||
             string.IsNullOrEmpty(txtdesc.Text) ||
             string.IsNullOrWhiteSpace(txtdesc.Text) ||
            string.IsNullOrEmpty(txtNPD_IPC_REFERENCE.Text) ||
             string.IsNullOrWhiteSpace(txtNPD_IPC_REFERENCE.Text) ||
            string.IsNullOrEmpty(NPDStatusPicker.SelectedItem.ToString()) ||
            string.IsNullOrWhiteSpace(NPDStatusPicker.SelectedItem.ToString()) ||
            string.IsNullOrEmpty(txtEnterPOS.Text) ||
             string.IsNullOrWhiteSpace(txtEnterPOS.Text) ||
            string.IsNullOrEmpty(txtNPD_REQ_QTY.Text) ||
            string.IsNullOrWhiteSpace(txtNPD_REQ_QTY.Text) ||
            string.IsNullOrEmpty(txtNPD_UOM.Text) ||
            string.IsNullOrWhiteSpace(txtNPD_UOM.Text) ||
            string.IsNullOrEmpty(txtNDP_Updated_By.Text) ||
            string.IsNullOrWhiteSpace(txtNDP_Updated_By.Text) ||
            string.IsNullOrEmpty(txtNPD_Inward.Text)||
            string.IsNullOrWhiteSpace(txtNPD_Inward.Text)
            )




        {
            DisplayAlert("message", "Please Fill the Fields", "OK");
        }

        else if (objmaterial != null)
        {
            EditMaterial();
        }
        else
        {
            AddMaterialData();
        }


}

private async void EditMaterial()
{
   // objmaterial.NPD_ALTERNATE_PART_NO = txtNPD_ALTERNATE_PART_NO.Text;

   await App.MyDababase.UpdateMaterial(objmaterial);
  // await Navigation.PushAsync(new NRCMeterialList(objmaterial));
}

private  async void AddMaterialData()
{


   await App.MyDababase.SaveMaterial(new Materials
    {
        PART_NO = txtNPD_ALTERNATE_OFFPART_NO.Text,
        NPD_ON_S_NO = txtOffSerialNumber.Text,
        NPD_ALTERNATE_PART_NO = txtonPart.Text,
        NPD_OFF_PART_NO = txtOnPArtNumbers.Text,
        NPD_REQ_ON_PART_NO = txtOnPArtNumbers.Text,
        NPD_Part_Type = txtPartType.Text,
        NPD_OFF_S_NO = txtNPD_OFF_S_NO.Text,

        NPD_DESC = txtdesc.Text,
        NPD_STATUS = NPDStatusPicker.SelectedItem.ToString(),
        NPD_IPC_REFERENCE = txtNPD_IPC_REFERENCE.Text,
        NPD_POS = txtEnterPOS.Text,
        NPD_REQ_QTY = txtNPD_REQ_QTY.Text,
        NPD_UOM = txtNPD_UOM.Text,
        NDP_Updated_By = txtNDP_Updated_By.Text,
        NPD_Inward = txtNPD_Inward.Text
    });

       await Navigation.PushAsync(new NRCMeterialList());
}