How can i declare null value for table fields in stored procedure

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C#

MySQL

hi this is stored procedure:

DELIMITER $$

CREATE DEFINER=`gps`@`%` PROCEDURE `SpAssetLost`(in HistoryID int,in AssetLostID int,in HistoryName varchar(250),in Remarks varchar(250),
in UserID int,in InMode varchar(200))

Begin
    

	if inmode= 'INSERT' then		
		insert into trnassetlost(HistoryID,HistoryName,Remarks,UserID)Values(HistoryID,HistoryName,Remarks,UserID);		
	end if;
    if inmode='UPDATE' then
        update trnassetlost set HistoryID=HistoryID,HistoryName=HistoryName,Remarks=Remarks,UserID=UserID where
        AssetLostID=AssetLostID;
    end if;
    if inmode='DELETE' then
         update trnassetlost set IsActive='FALSE' where AssetLostID=AssetLostID;
    end if;
    if inmode='SELECT' then
        select * from trnassetlost;
    end if;

end

I was wrote select function in my class file using c#:

 public Access objAccess = new Access();

 public DataSet SELECT(BALAssetLost ObjAl)
        {
            param = new MySqlParameter[1];
            param[0] = new MySqlParameter("@inmode", "SELECT");

            DataSet ds = objAccess.GetDataSet(param, "SpAssetLost");
            return ds;
        }

Access.cs:
 public DataSet GetDataSet(MySqlParameter[] param, string procedure)
        {
            try
            {
                establishConnection();
                try
                {
                    if (con.State == ConnectionState.Open)
                    {
                        con.Close();

                    }
                    con.Open();
                    MySqlCommand cmd = new MySqlCommand(procedure, con);
                    cmd.CommandType = CommandType.StoredProcedure;
                    cmd.Parameters.AddRange(param);
                    MySqlDataAdapter da = new MySqlDataAdapter(cmd);
                    DataSet ds = new DataSet();
                    da.Fill(ds);
                    return ds;
                }
                catch (Exception ex)
                {
                    throw ex;
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

now i got error "Parameter 'HistoryID' not found in the collection".so i want to set null in unwanted variables.how can i declare null while creating stored procedure in mysql workbench.

PLEASE ANYBODY HELP ME.
Please anybody help me.