MultiBinding for Picker's ItemDisplayBinding throws Null reference exception on adding new item

Question

Consider following XAML in Xamarin Forms. I have binded a Picker control with an ObserableCollection and assigned a MultiBinding to its ItemDisplayBinding property -

<Picker ItemsSource="{Binding BooksList}">
   <Picker.ItemDisplayBinding>
       <MultiBinding StringFormat="{}{0} - {1}">
           <Binding Path="Title"/>
           <Binding Path="Subtitle"/>
       </MultiBinding>
   </Picker.ItemDisplayBinding>
</Picker>

<Button Text="Add" Command="{Binding AddCommand}"/>
<Button Text="Remove" Command="{Binding RemoveCommand}"/>

And the corresponding ViewModel -

AddCommand = new Command((object item) =>
{
    BooksList.Add(new ListItem
            {
                Title = "Book Name",
                Subtitle = "Author Name"
            });
}
...

private ObservableCollection<ListItem> _booksList;
public ObservableCollection<ListItem> BooksList
{
    get => _booksList;
    set
    {
        _booksList = value;
        OnPropertyChanged(nameof(BooksList));
    } 
}

...

public BookClass
{
    BooksList = new ObservableCollection<ListItem>();
}

...

public class ListItem
{
    public string Title { get; set; }
    public string Subtitle { get; set; }
}

Now, when I run the application and hit the Add button ONCE it works fine and I can see an element in the picker control. However, when I hit the Add button again, it throws following exception -

Object reference not set to an instance of an object

Stack trace -

{System.NullReferenceException: Object reference not set to an instance of an object.
  at Xamarin.Forms.MultiBinding.Apply (System.Object context, Xamarin.Forms.BindableObject targetObject, Xamarin.Forms.BindableProperty targetProperty, System.Boolean fromBindingContextChanged) [0x0013a] in D:\a\1\s\Xamarin.Forms.Core\MultiBinding.cs:147 
  at Xamarin.Forms.Picker.GetDisplayMember (System.Object item) [0x00018] in D:\a\1\s\Xamarin.Forms.Core\Picker.cs:184 
  at Xamarin.Forms.Picker.AddItems (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00032] in D:\a\1\s\Xamarin.Forms.Core\Picker.cs:252 
  at Xamarin.Forms.Picker.CollectionChanged (System.Object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00010] in D:\a\1\s\Xamarin.Forms.Core\Picker.cs:238 
  at System.Collections.ObjectModel.ObservableCollection`1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedEventArgs e) [0x00018] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/System.ObjectModel/src/System/Collections/ObjectModel/ObservableCollection.cs:263 
  at System.Collections.ObjectModel.ObservableCollection`1[T].OnCollectionChanged (System.Collections.Specialized.NotifyCollectionChangedAction action, System.Object item, System.Int32 index) [0x00000] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/System.ObjectModel/src/System/Collections/ObjectModel/ObservableCollection.cs:338 
  at System.Collections.ObjectModel.ObservableCollection`1[T].InsertItem (System.Int32 index, T item) [0x0001a] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/System.ObjectModel/src/System/Collections/ObjectModel/ObservableCollection.cs:196 
  at System.Collections.ObjectModel.Collection`1[T].Add (T item) [0x00020] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/Common/src/CoreLib/System/Collections/ObjectModel/Collection.cs:71 
  at RS.Apps.ViewModels.BookShelf.BookShelfViewModel.<.ctor>b__37_3 (System.Object item) [0x00018] in C:\user43424\GitHub\Booking\Mobile\Apps\ViewModels\BookShelf\BookShelfViewModel.cs:97 }

I am unable to wrap my hand around in what's going on here. Why the null reference exception, what am I missing?