Xamarin – Fill ListView with MySql Data

I’m kind of stuck and not getting anywhere. Here is my problem:

I want to populate a ListView with the data from a MySql database. Here is my class which contains the database queries:

DBConnection:

public ObservableRangeCollection<ExpenseReport> GetDataExpenseReport()
        {
            if (OpenConnection() == true)
            {
                DataExpense = new ObservableRangeCollection<ExpenseReport>();

                MySqlCommand sc = new MySqlCommand();
                sc.CommandText = "SELECT * FROM t_expensereport WHERE c_inputuser = @userid";
                sc.Parameters.AddWithValue("@userid", User.Id);
                sc.Connection = myConn;

                MySqlDataReader dr = sc.ExecuteReader();

                while (dr.Read())
                {
                    DataExpense.Add(new ExpenseReport
                    {
                        Date = (DateTime)dr["c_date"],
                        Reason = (string)dr["c_reason"],
                        TripThereAndBack = (string)dr["c_tripThereAndBack"],
                        Vehicle = (string)dr["c_vehicle"],
                        Kilometers = (double)dr["c_kilometers"],
                        EuroProKm = (double)dr["c_euroProKm"],
                        TripCost = (double)dr["c_tripCost"],
                        Maut = (double)dr["c_maut"],
                        ParkingLot = (double)dr["c_parkingLot"],
                        Other = (double)dr["c_other"],
                        Accommodation = (double)dr["c_accommodation"],
                        Meal = (double)dr["c_meal"],
                        TrainTicket = (double)dr["c_trainTicket"]
                    });

                }
                dr.Close();
                return DataExpense;
            }
            return null;

        }

My Model ExpenseReport:

public class ExpenseReport
{
  public DateTime Date { get; set; }
  public string Reason { get; set; }
  public string TripThereAndBack { get; set; }
  public string Vehicle { get; set; }
  public double Kilometers { get; set; }
  public double EuroProKm { get; set; }
  public double TripCost { get; set; }
  public double Maut { get; set; }
  public double ParkingLot { get; set; }
  public double Other { get; set; }
  public double Accommodation { get; set; }
  public double Meal { get; set; }
  public double TrainTicket { get; set; }
}

View:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:viewmodels="clr-namespace:Magazin.ViewModels.Dashboard"
             xmlns:xct="http://xamarin.com/schemas/2020/toolkit" 
             xmlns:model="clr-namespace:Magazin.Models"
             x:Class="Magazin.Views.Dashboard.OverviewExpenseReportView"
             Title="Übersicht Spesen">
    <ContentPage.BindingContext>
        <viewmodels:OverviewExpenseReportViewModel/>
    </ContentPage.BindingContext>
    <ContentPage.Behaviors>
        <xct:EventToCommandBehavior
                EventName="Appearing"
                Command="{Binding RefreshCommand}" />
    </ContentPage.Behaviors>
    <ContentPage.Content>
        <StackLayout>
            <ListView
                x:Name="OverviewER"
                CachingStrategy="RecycleElement"
                BackgroundColor="Transparent"
                ItemsSource="{Binding ExpenseReport}"
                HasUnevenRows="True"
                SeparatorVisibility="None"
                IsPullToRefreshEnabled="True"
                IsRefreshing="{Binding IsBusy, Mode=OneWay}"
                RefreshControlColor="Green"
                RefreshCommand="{Binding RefreshCommand}">
                <!---->



                <ListView.ItemTemplate>
                    <DataTemplate x:DataType="model:ExpenseReport">
                        <ViewCell>
                            <Grid Padding="10">
                                <Frame CornerRadius="20" HasShadow="True">
                                    <StackLayout Orientation="Horizontal">
                                        <StackLayout Padding="20,0,0,0">
                                            <StackLayout>
                                                <Label Text="{Binding Reason}" FontSize="Medium" VerticalOptions="Center" TextColor="Green" />
                                            </StackLayout>
                                            <StackLayout>
                                                <Label Text="{Binding Kilometers, StringFormat='{}{0}km'}" FontSize="Small" VerticalOptions="Center"/>
                                                <Label Text="{Binding Date, StringFormat='{0:dd.MM.yyyy}'}" FontSize="Small" VerticalOptions="Center"/>
                                                <Label Text="{Binding TripCost}" FontSize="Small" VerticalOptions="Center"/>
                                            </StackLayout>
                                        </StackLayout>
                                    </StackLayout>
                                </Frame>
                            </Grid>

                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

Viewmodel OverviewExpenseReportViewModel

public class OverviewExpenseReportViewModel : ViewModelBase
    {
        public ObservableRangeCollection<ExpenseReport> ExpenseReportCollection { get; set; }
        public AsyncCommand RefreshCommand { get; }
        public OverviewExpenseReportViewModel()
        {
            RefreshCommand = new AsyncCommand(Refresh);
        }
        async Task Refresh()
        {
            ExpenseReportCollection = new ObservableRangeCollection<ExpenseReport>();
            DBConnection db = new DBConnection();
            await Task.Delay(1500);

            ExpenseReportCollection.Clear();
            ExpenseReportCollection.AddRange(db.GetDataExpenseReport());

        }
    }

I have long asked our friend Google but I somehow do not get anywhere. At this point
ExpenseReport.AddRange(db.GetDataExpenseReport()); I get the error that System.NullReferenceException: 'Object reference not set to an instance of an object.'

Can anyone help me on how I manage to populate the ListView, I appreciate any answers!

Lastly, I switched everything to ObservableRangeCollection from the MvvmHelpers dependency. But I always get this error, something I am doing wrong.