StackPanelPage Error when RadioButton_Checked is called

### Which version of the app?

WinUI 3 Gallery

### Description

There is an error on StackPanelPage.xaml.cs when this is called for the first time:

Original Line 26:

string orientationName = rb.Tag.ToString();

Updated Line:

string orientationName = rb.Content.ToString();

### Screenshots




### Windows version

_No response_

### Additional context

_No response_

Open StackPanelPage.xaml.cs and inspect RadioButton_Checked around line 26. Reproduce the first invocation in WinUI 3 Gallery, check the radio button's available values, and verify the handler no longer errors when first called.

Written by the indexing model from the issue text.