ThemeResource failed to assign to custom property

### Describe the bug

I have a custom templated control, and I want to bind its property to a `ThemeResource`, and I got crash at xaml loading phase during app launch.

### Steps to reproduce the bug

1. Create a new WinUI3 C++ packaged project
2. Create a new templated control, with this idl
```idl
[default_interface]
runtimeclass MyControl : Microsoft.UI.Xaml.Controls.Control
{
MyControl();
String StringProperty;
}
```
3. In `MainWindow.xaml`, use this code
```xml





Light


Dark

```
4. Build and run, see the crash

### Expected behavior

_No response_

### Screenshots


### NuGet package version

WinUI 3 - Windows App SDK 1.6 Preview 1: 1.6.240807006-preview1

### Windows version

Windows 11 (22H2): Build 22621

### Additional context
Making it to a `DependencyProperty` does not solve this issue.
I tried and this issue also happens in UWP too.
[Repro](https://github.com/HO-COOH/WinUI-Bug/tree/main/15-CustomThemeResourceException)

Start by running the linked 15-CustomThemeResourceException reproduction and inspect MainWindow.xaml, the MyControl IDL, and the ThemeResource assignment during XAML loading. Done means the custom control loads without a crash and StringProperty resolves to the correct Light or Dark resource.

Written by the indexing model from the issue text.