I have problems fetching data form one form to another

I have problems fetching data form one form to another

When I tired to fetch data form one form to another de my application did not work.

I put the followin script in the look up field and the application dont work

if (input.Principal  !=  null)
{
    temp  =  Principal  [Id_de_Voluntario == input.Principal];
    input.Estatus1 = temp.Estatus;
}

Heeeelllpppppp