Merge pull request #1114 from orrindeng/pr230313

Fix combobox can't open dropdown when the editable textbox of it has the focus

Author: batzen

Fluent.Ribbon/Controls/ComboBox.cs

@@ -658,7 +658,11 @@ protected override void OnDropDownOpened(EventArgs e)
 
         if (this.SelectedItem is not null)
         {
-            Keyboard.Focus(this.ItemContainerGenerator.ContainerOrContainerContentFromItem<IInputElement>(this.SelectedItem));
+            var selectedItemContainer = this.ItemContainerGenerator.ContainerOrContainerContentFromItem<IInputElement>(this.SelectedItem);
+            if (selectedItemContainer is not null)
+            {
+                Keyboard.Focus(selectedItemContainer);
+            }
         }
 
         this.focusedElement = Keyboard.FocusedElement;