Guys, why is the event null?

Question

WPF

Sori that a lot of information))
There is a MainWindows form
It has a button that calls the AddContent Form

private void ButtonAddSpending_Click(object sender, RoutedEventArgs e) {          
            AddContent addContent = new AddContent();
            addContent.Owner = this;
            addContent.ShowDialog();
        }

The AddContent form has an event when the form is called, in the constructor I subscribe to the event. here is ButtonAddContent_Click itselfpublic event EventHandler EventAddContent;

buttonAddContent.Click += ButtonAddContent_Click;

private void ButtonAddContent_Click(object sender, RoutedEventArgs e) {
            EventAddContent?.Invoke(this, EventArgs.Empty);
            this.Activate();
            buttonAddContent.IsEnabled = false;
        }

There is a class that controls all movements in the application
class MainPresenter
{
private readonly IAddContentPresenter _addContent;
in the constructor in the constructor I pass the interface in which there is this event.
(Form implements this interface)

public MainPresenter( IAddContentPresenter addContent) {           
            _addContent = addContent;
            _addContent.EventAddContent += View_AddContent;
        }

this class already has a method that implements pressing

private void View_AddContent(object sender, EventArgs e) { //Метод записывает данные в БД
            _manager.SetDBHistories(_history);
        }

Class constructor called in MainWindow
event not working!
in the debugger the event shows

null
 EventAddContent?.Invoke(this, EventArgs.Empty);