I’m working on a project (ASP.NET MVC5) for school but today I ran into this error even the same code yesterday is exactly like this 100% but still ran and today when i open it again it run to error. I’m already trying to fix this for a long hours but nothing works. I don’t know why it null anyway, i have tried to fix a lot but still get null when i put the edit ( SuaCT in the CT View ), and also the Details ( ChiTietCT ) and Delete ( XoaCT ) is get null errors too, its seem when i click on these buttons, it can not get the data from the list to edit, details and delete.