feat: throws when collection is not initialized.

demo/Ursa.Demo/Pages/MultiAutoCompleteBoxDemo.axaml

@@ -17,12 +17,10 @@
                                 FilterMode="Custom">
             <u:MultiAutoCompleteBox.ItemTemplate>
                 <DataTemplate DataType="vm:ControlData">
-                    <TextBlock>
-                        <Run Text="{Binding MenuHeader}" />
-                        <Run Text="(" />
-                        <Run Text="{Binding Chinese}" />
-                        <Run Text=")" />
-                    </TextBlock>
+                    <StackPanel Orientation="Horizontal">
+                        <TextBlock Text="{Binding MenuHeader}" VerticalAlignment="Center"/>
+                        <TextBlock Text="{Binding Chinese}" Classes="Secondary" FontSize="12" Margin="8 0 0 0" VerticalAlignment="Center"/>
+                    </StackPanel>
                 </DataTemplate>
             </u:MultiAutoCompleteBox.ItemTemplate>
             <u:MultiAutoCompleteBox.SelectedItemTemplate>

src/Ursa/Controls/AutoCompleteBox/MultiAutoCompleteBox.cs

@@ -1587,6 +1587,7 @@ public partial class MultiAutoCompleteBox : TemplatedControl, IInnerContentContr
 
         // Completion will update the selected value
         //UpdateTextCompletion(false);
+        _ = SelectedItems ?? throw new NullReferenceException("Selected Items collection must be initialized. ");
         SelectedItems?.Add(_adapter?.SelectedItem);
         UpdateTextValue(string.Empty, false);