✅ Avalonia not showing rows in datagrid

ObservableCollection<BEURTINFO> BeurtenGRID {get;} = new ObservableCollection<BEURTINFO>();//in top of code
//in function
for (int i = 0; i < jsonObj.Length; i++)//jsonObj is the parsed JSON string from server
{
BeurtenGRID.Add(new BEURTINFO( jsonObj[i].username, formatTime(Convert.ToInt32(jsonObj[i].time)).ToString(), jsonObj[i].devices.ToString()));
}
//bottom of code
public class BEURTINFO
{
 public string Username { get; set; }
 public string Time { get; set; }
 public string Devices { get; set; }
 public BEURTINFO(string username, string time, string devices) { 
  Username = username;
  Time = time; 
  Devices = devices; 
 }
}

<Design.DataContext>
<vm:MainViewModel></vm:MainViewModel>
</Design.DataContext>
<DataGrid Loaded="DataGrid_Loaded"
      ItemsSource="{Binding BeurtenGRID}"
      x:Name="beurtenGrid"
      Margin="20"
      IsReadOnly="False"
      CanUserReorderColumns="False"
      CanUserResizeColumns="False"
      CanUserSortColumns="True"
      GridLinesVisibility="All"
      BorderThickness="1" BorderBrush="Gray">
        <DataGrid.Columns>
            <DataGridTextColumn Header="Gebruikersnaam"  Binding="{Binding Username}" Width="300" />
                        <DataGridTextColumn Header="Duratief/Data" Binding="{Binding Time}" Width="200"/>
                        <DataGridTextColumn Header="Aantal apparaten" Binding="{Binding Devices}" Width="200">
                </DataGrid.Columns>
</DataGrid>

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="GIP_av.App"
             RequestedThemeVariant="Light">
             <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->

    <Application.Styles>
        <FluentTheme />
        <StyleInclude Source="avares://Avalonia.Controls.DataGrid/Themes/Fluent.xaml"/>
    </Application.Styles>
</Application>

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="GIP_av.BeurtenBekijken"
        xmlns:vm="using:GIP_av.ViewModels"
        Title="BeurtenBekijken"
        ExtendClientAreaToDecorationsHint="True"
        ExtendClientAreaChromeHints="NoChrome"
        ExtendClientAreaTitleBarHeightHint="-1"
        WindowState="Maximized"
        SystemDecorations="BorderOnly">
    <StackPanel>
        <Design.DataContext>
            <vm:MainViewModel></vm:MainViewModel>
        </Design.DataContext>
        <DataGrid Loaded="DataGrid_Loaded"
      ItemsSource="{Binding BeurtenGRID}"
      x:Name="beurtenGrid"
      Margin="20"
      IsReadOnly="False"
      CanUserReorderColumns="False"
      CanUserResizeColumns="False"
      CanUserSortColumns="True"
      GridLinesVisibility="All"
      BorderThickness="1" BorderBrush="Gray"
                  Height="300">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Gebruikersnaam"  Binding="{Binding Username}"
                                    Width="300" Foreground="Black" />
                <DataGridTextColumn Header="Duratief/Data" Binding="{Binding Time}" 
                                    Width="200"/>
                <DataGridTextColumn Header="Aantal apparaten" Binding="{Binding Devices}" 
                                    Width="200"/>
            </DataGrid.Columns>
        </DataGrid>
        <Button
    Click="Sluiten_Click"
    FontSize="18"
    HorizontalAlignment="Center"
    VerticalAlignment="Bottom"
            >Sluiten</Button>
    </StackPanel>
</Window>

<StyleInclude Source="avares://Avalonia.Controls.DataGrid/Themes/Fluent.xaml" />