WPF ComboBox text search causes NullReferenceException with the toggle button's IsChecked property

Question

I've hit a problem with WPF combo boxes and changing the data source after the user has pressed characters that don't appear in the data.

<Label Name="FavoriteFoodLbl" Grid.Column="0" Grid.Row="13">Favorite Food</Label>
<ComboBox Name="FavoriteFoodCombo" Grid.Column="1" Grid.Row="13" ItemsSource="{Binding Foods}" SelectedItem="{Binding FavoriteFood, UpdateSourceTrigger=PropertyChanged}" />

<Label Name="FavoriteFlavourLbl" Grid.Column="2" Grid.Row="13">Favorite Flavour</Label>
<ComboBox Name="FavoriteFlavourCombo" Grid.Column="3" Grid.Row="13" ItemsSource="{Binding Flavours}"  />

Code....

    public ObservableCollection<string> Foods { get; set; } = new ObservableCollection<string>() { "Pizza", "Ice Cream", "Soup" };

    private string _favoriteFood;
    public string FavoriteFood
    {
        get { return _favoriteFood; }
        set
        {
            _favoriteFood = value;
            switch (_favoriteFood)
            {
                case "Pizza":
                    _flavours = new ObservableCollection<string>(PizzaToppings);
                    break;
                case "Ice Cream":
                    _flavours = new ObservableCollection<string>(IceCreamFlavours);
                    break;
                case "Soup":
                    _flavours = new ObservableCollection<string>(SoupFlavours);
                    break;
                default:
                    _flavours = new ObservableCollection<string>(new List<string>() { "None" });
                    break;
            }
            this.FavoriteFlavour = null;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("FavoriteFood"));
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Flavours"));
        }
    }

    public List<string> PizzaToppings { get; set; } = new List<string>() { "Margarita", "Pepperoni", "Meat Feast" };
    public List<string> IceCreamFlavours { get; set; } = new List<string>() { "Vanilla", "Strawberry", "Chocolate" };
    public List<string> SoupFlavours { get; set; } = new List<string>() { "Tomato", "Leek and Potato", "Chicken" };

    private ObservableCollection<string> _flavours = null;
    public ObservableCollection<string> Flavours
    {
        get
        {
            return _flavours;
        }
        set
        {
            _flavours = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Flavours"));
        }
    }

    string _favoriteFlavour;
    public string FavoriteFlavour
    {
        get
        {
            return _favoriteFlavour;
        }

        set
        {
            _favoriteFlavour = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("FavoriteFlavour"));
        }
    }

The problem only occurs when the user selects the value in the second combo box by typing (rather than via a mouse click). Steps to reproduce:

1. Select a favorite food e.g. pizza
2. Use the tab key to move to the second combo
3. Press the first letter of a known option e.g. press 'P' for "Pepperoni"
4. Press another letter for which there are no options starting with it i.e. press 'e', as if you were still typing "Pepperoni"
5. Now choose a different favorite food e.g. Ice Cream. The value "Pepperoni" should disappear from the second box.
6. Now with the mouse, try to choose a favorite flavour of ice cream.

At this point I get an exception being thrown...

Activated   Event   Time    Duration    Thread
Cannot save value from target back to source. BindingExpression:Path=IsDropDownOpen; DataItem='ComboBox' (Name='FavoriteFlavourCombo'); target element is 'ToggleButton' (Name='toggleButton'); target property is 'IsChecked' (type 'Nullable`1') NullReferenceException:'System.NullReferenceException: Object reference not set to an instance of an object.
at System.Windows.Controls.ComboBox.CoerceIsSelectionBoxHighlighted(Object o, Object value)
at System.Windows.DependencyObject.ProcessCoerceValue(DependencyProperty dp, PropertyMetadata metadata, EntryIndex& entryIndex, Int32& targetIndex, EffectiveValueEntry& newEntry, EffectiveValueEntry& oldEntry, Object& oldValue, Object baseValue, Object controlValue, CoerceValueCallback coerceValueCallback, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, Boolean skipBaseValueChecks)
at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
at System.Windows.DependencyObject.CoerceValue(DependencyProperty dp)
at System.Windows.Controls.ComboBox.OnIsDropDownOpenChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType, Boolean isInternal)
at MS.Internal.Data.PropertyPathWorker.SetValue(Object item, Object value)
at MS.Internal.Data.ClrBindingWorker.UpdateValue(Object value)
at System.Windows.Data.BindingExpression.UpdateSource(Object value)'    28.20s      

I have uploaded a sample to github: https://github.com/DanMcCoy/ComboBoxBugDemo This sample uses framework 4.6.1. I am using Visual Studio Enterprise 2015 (14.0.25425.01 Update 3).

There's a video in the GitHub repo demonstrating the problem.