Saving WPF DataGrid changes to Entity Framework
Other issues...


C# variable naming convention is camelCase. And definitely don't start a local's name with an underscore, as underscores are understood to prefix instance variables.
There's no separation of concerns here. Your UI code is talking to the database. That's not the responsibility of the UI.
I assume ApplicationDbContext inherits from DbContext which is implements IDisposable. You're not disposing it. Use it in a using block. (I know nothing about Entity Framework so don't know if there's a good reason not to do so.)