Application crash when using the x:Bind in ComboBox

### Describe the bug

In ComboBox, `SelectedItem` property is binded with x:bind and the combo box is editable. Now, user tried to enter value manually and press enter. Application gets crashed.

Note: If we use Binding the control works fine.

[MSControlTest.zip](https://github.com/user-attachments/files/19703636/MSControlTest.zip)

### Steps to reproduce the bug

1. Run the sample.
2. Edit the combo box and enter the non-listed item.
3. Now, press enter key.

Application crashed with Unhandled exception.

### Expected behavior

It should work fine with all type of binding.

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.7.1: 1.7.250401001

### Windows version

Windows 11 (23H2): Build 22631

### Additional context

_No response_

Start by running the attached MSControlTest.zip sample with Windows App SDK 1.7.1 on Windows 11 23H2, then reproduce the crash by entering a non-listed item in the editable ComboBox and pressing Enter. Compare the x:Bind and Binding cases; done means the x:Bind case no longer crashes and the control continues to work with manually entered values.

Written by the indexing model from the issue text.