如何解决：对象引用未设置为对象的实例

Question

对象引用未设置为对象的实例。遇到此异常，我该如何解决此问题。

              public bool Remove(/*int id*/UserMaster item)
              {
         con = null;
        using (SqlConnection sqlCon = new SqlConnection(connectionstring))
            {
              SqlCommand cmd = new SqlCommand("spDeleteEmployee", con);
              cmd.CommandType = CommandType.StoredProcedure;

              SqlParameter paramID = new SqlParameter();
              paramID.ParameterName = "@ID";
              paramID.Value =item.ID;
              cmd.Parameters.Add(paramID);
 ERROR LINE:  con.Open();
              cmd.ExecuteNonQuery();
              }