Remove parent ObservableCollection in C#

Question

I have 2 classes, Fleet and Car, a Fleet can have multiple Cars, both classes are ObservableCollections.

I made a button which has to delete the selected Fleet in a ListBox, when I click the button:

    private void btRemoveFleet_Click(object sender, RoutedEventArgs e)
    {
        Fleets.Remove((Fleet)lbFleets.SelectedItem);
    }

When I click the button in the following code the error occurs:

    private void lbFleets_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
    {
        Cars = ((Fleet)lbFleets.SelectedItem).Car;
        lbCars.ItemsSource = Cars;

        grdCarOverview.DataContext = Cars;
    }

     private void lbFleets_SelectionChanged(object sender, System.Windows.Controls.SelectionChangedEventArgs e)
    {
        Cars = ((Fleet)lbFleets.SelectedItem).Car;
        lbCars.ItemsSource = Cars;

        grdCarOverview.DataContext = Cars;
    }

The error I get

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

I'm kind of new to C# and would just want to know what's going on and how to fix it.