Object Reference Not Set To An Instance Of An Object

Na minha pagina aspx, ta com erro

Server Error in '/' Application.
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 27:         {
Line 28:
Line 29:              corpo = "<table>" +
Line 30:             "<tr><td> <b>Empresa:</b></td><td>" + item.empresa.ToString() + "</td></tr>" +
Line 31:              "<tr><td> <b>CNPJ:</b></td><td>" + item.cnpj.ToString() + "</td></tr>" +


Source File: c:\inetpub\vhosts\core-go.org.br\httpdocs\enviaroferta2.aspx.cs    Line: 29

no banco de dados ta dando insert so que ta com poblema no envio para o email, alguem sabe resolver