I wanted to know if I was wrong in creating the USP ConsultBook or in business entities

  

When I want to update a book, the combobox of the Book goes blank (supposedly I should leave with the data of the book) but when I display the list of authors, categories, etc, and when I change the year of publication or Title of the book I get that message: / please someone who can help me.

THIS IS IN THE LAYER ACCESS TO DATA

public LibroBE ConsultarLibro(String strcod)
    {
        LibroBE objLibroBE = new LibroBE();
        try
        {
            //Codifique
            cnx.ConnectionString = MiConexion.GetCnx();
            cmd.Connection = cnx;
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.CommandText = "usp_ConsultarLibro";

            cmd.Parameters.AddWithValue("@id_Libro", strcod);
            cnx.Open();
            dtr = cmd.ExecuteReader();

            if (dtr.HasRows == true)
            {
                dtr.Read();

                if (dtr["IdLibro"] == DBNull.Value)
                {
                    objLibroBE.Id_Libro = "";
                }
                else
                {
                    objLibroBE.Id_Libro = dtr["IdLibro"].ToString();
                }


                if (dtr["Titulo"] == DBNull.Value)
                {
                    objLibroBE.Tit_Libro = "";
                }
                else
                {
                    objLibroBE.Tit_Libro = dtr["Titulo"].ToString();
                }


                if (dtr["NomApe_Autor"] == DBNull.Value)
                {
                    objLibroBE.Id_Autor = "";
                }
                else
                {
                    objLibroBE.Id_Autor = dtr["NomApe_Autor"].ToString();
                }


                if (dtr["Nombre_Categoria"] == DBNull.Value)
                {
                    objLibroBE.Id_Categoria = "";
                }
                else
                {
                    objLibroBE.Id_Categoria = dtr["Nombre_Categoria"].ToString();
                }

                if (dtr["Nombre_Editorial"] == DBNull.Value)
                {
                    objLibroBE.Id_Editorial = "";
                }
                else
                {
                    objLibroBE.Id_Editorial = dtr["Nombre_Editorial"].ToString();
                }

                if (dtr["Nombre_Idioma"] == DBNull.Value)
                {
                    objLibroBE.Id_Idioma = "";
                }
                else
                {
                    objLibroBE.Id_Idioma =  dtr["Nombre_Idioma"].ToString();
                }

                if (dtr["Año_Lanz"] == DBNull.Value)
                {
                    objLibroBE.Año_Lanz = "";
                }
                else
                {
                    objLibroBE.Año_Lanz = dtr["Año_Lanz"].ToString();
                }

THIS IS IN BUSINESS ENTITIES

public class LibroBE
{
    private String mvarId_Libro;
    public String Id_Libro
    {
        get { return mvarId_Libro; }
        set { mvarId_Libro = value; }
    }
    private String mvartit_Libro;
    public String Tit_Libro
    {
        get { return mvartit_Libro; }
        set { mvartit_Libro = value; }
    }
    private String mvarId_Autor;
    public String Id_Autor
    {
        get { return mvarId_Autor; }
        set { mvarId_Autor = value; }
    }

    private String mvarId_Categoria;
    public String Id_Categoria
    {
        get { return mvarId_Categoria; }
        set { mvarId_Categoria = value; }
    }

    private String mvarId_Editorial;
    public String Id_Editorial
    {
        get { return mvarId_Editorial; }
        set { mvarId_Editorial = value; }
    }

    private String mvarId_Idioma;
    public String Id_Idioma
    {
        get { return mvarId_Idioma; }
        set { mvarId_Idioma = value; }
    }

    private String mvarAño_Lanz;
    public String Año_Lanz
    {
        get { return mvarAño_Lanz; }
        set { mvarAño_Lanz = value; }
    }

THIS IN THE CODE OF FORMS