Bind scheduler to XPO in devexpress

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C#

WPF

DevExpress

I need to bind my scheduler to devexpress xpo in WPF the problem is I don't know how to use XPCollection in this case..

1- I used the wizard to automatically generate a data model from the database
2- I have a scheduler with mappings in XAML
3- In code-behind, this is what I used and I think the problem is here

public MainWindow()  
        {  
  
            InitializeComponent();  
  
  
            XPCollection Appointmentcollection = new XPCollection(typeof(MedicalAppointments));  
            scheduler.DataSource.AppointmentsSource = Appointmentcollection;  
  
            XPCollection Doctorscollection = new XPCollection(typeof(Doctors));  
            scheduler.DataSource.ResourcesSource = Doctorscollection;  
        }

In Debug, whenever I try to add an appointment by double-clicking a field in scheduler control or by using the ribbon I get an error:

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

How can I fix this and what is the best way ti initiate an XPCollection In Code?

What I have tried:

public MainWindow()  
        {  
  
            InitializeComponent();  
  
  
            XPCollection Appointmentcollection = new XPCollection(typeof(MedicalAppointments));  
            scheduler.DataSource.AppointmentsSource = Appointmentcollection;  
  
            XPCollection Doctorscollection = new XPCollection(typeof(Doctors));  
            scheduler.DataSource.ResourcesSource = Doctorscollection;  
        }