Sporadic crash in `AvTrace.IsWpfTracingEnabledInRegistry()` when calling `PresentationTraceSources.Refresh()` from non-UI thread

### Description

Calling `PresentationTraceSources.Refresh()` from a thread other than the UI thread may sporadically lead to `System.InvalidOperationException: Nullable object must have a value.` in `AvTrace.IsWpfTracingEnabledInRegistry()` (see two possible stack traces below).

The documentation of `PresentationTraceSources.Refresh()` does not mention anything about having to be called on a specific thread. And looking at [the source code implicated by the stack traces](https://github.com/dotnet/wpf/blob/f2a578639ada4f5d9b06d701e657667b4ec9c634/src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/AvTrace.cs#L217-L241), it seems like the class is intended to be thread-safe, in particular because of the comment in `IsWpfTracingEnabledInRegistry()`:

```csharp
if( _enabledInRegistry == null )
{
[...]

// Update the static. Doing this last protects us from threading problems; worse case, multiple
// threads will set the same value into it.
_enabledInRegistry = enabled;
}

return (bool) _enabledInRegistry;
```

However turns out this comment isn't quiet correct: Doing it twice isn't the worst case. A thread other than the UI thread may call `PresentationTraceSources.Refresh()`, which in turn calls `AvTrace.Refresh()`. If this happens between the assignment (`_enabledInRegistry = enabled`) and the return statement then the `Nullable _enabledInRegistry` will not have a value and the cast to `bool` throws an exception.

```csharp
public void Refresh()
{
// Cause the registry to be re-read in case it's changed.
_enabledInRegistry = null;

// Re-initialize everything
Initialize();
}
```

Stack traces

```
Unhandled exception. System.Windows.Markup.XamlParseException: 'Initialization of 'System.Windows.Controls.TextBlock' threw an exception.' Line number '9' and line position '76'.
---> System.Xaml.XamlObjectWriterException: Set property 'System.Windows.ResourceDictionary.DeferrableContent' threw an exception.
---> System.TypeInitializationException: The type initializer for 'MS.Internal.TraceResourceDictionary' threw an exception.
---> System.InvalidOperationException: Nullable object must have a value.
at System.Nullable`1.get_Value()
at MS.Internal.AvTrace.IsWpfTracingEnabledInRegistry()
at MS.Internal.AvTrace.ShouldCreateTraceSources()
at MS.Internal.AvTrace.Initialize()
at MS.Internal.TraceResourceDictionary..cctor()
--- End of inner exception stack trace ---
at MS.Internal.TraceResourceDictionary.get_IsEnabled()
at System.Windows.ResourceDictionary.SetKeys(IList`1 keyCollection, IServiceProvider serviceProvider)
at MS.Internal.Xaml.Runtime.ClrObjectRuntime.SetValue(Object inst, XamlMember property, Object value)
--- End of inner exception stack trace ---
at MS.Internal.Xaml.Runtime.ClrObjectRuntime.SetValue(Object inst, XamlMember property, Object value)
at MS.Internal.Xaml.Runtime.PartialTrustTolerantRuntime.SetValue(Object obj, XamlMember property, Object value)
at System.Xaml.XamlObjectWriter.Logic_ApplyPropertyValue(ObjectWriterContext ctx, XamlMember prop, Object value, Boolean onParent)
at System.Xaml.XamlObjectWriter.Logic_DoAssignmentToParentProperty(ObjectWriterContext ctx)
at System.Xaml.XamlObjectWriter.WriteEndMember()
at System.Xaml.XamlServices.Transform(XamlReader xamlReader, XamlWriter xamlWriter, Boolean closeWriter)
at System.Windows.SystemResources.ResourceDictionaries.LoadDictionary(Assembly assembly, String assemblyName, String resourceName, Boolean isTraceEnabled, Uri& dictionarySourceUri)
at System.Windows.SystemResources.ResourceDictionaries.LoadThemedDictionary(Boolean isTraceEnabled)
at System.Windows.SystemResources.FindDictionaryResource(Object key, Type typeKey, ResourceKey resourceKey, Boolean isTraceEnabled, Boolean allowDeferredResourceReference, Boolean mustReturnDeferredResourceReference, Boolean& canCache)
at System.Windows.SystemResources.FindResourceInternal(Object key, Boolean allowDeferredResourceReference, Boolean mustReturnDeferredResourceReference)
at System.Windows.StyleHelper.GetThemeStyle(FrameworkElement fe, FrameworkContentElement fce)
at System.Windows.FrameworkElement.UpdateThemeStyleProperty()
at System.Windows.FrameworkElement.OnInitialized(EventArgs e)
at MS.Internal.Xaml.Runtime.ClrObjectRuntime.InitializationGuard(XamlType xamlType, Object obj, Boolean begin)
--- End of inner exception stack trace ---
at System.Windows.Markup.XamlReader.RewrapException(Exception e, IXamlLineInfo lineInfo, Uri baseUri)
at System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, IXamlObjectWriterFactory writerFactory, Boolean skipJournaledProperties, Object rootObject, XamlObjectWriterSettings settings, Uri baseUri)
at System.Windows.Markup.WpfXamlLoader.LoadBaml(XamlReader xamlReader, Boolean skipJournaledProperties, Object rootObject, XamlAccessLevel accessLevel, Uri baseUri)
at System.Windows.Markup.XamlReader.LoadBaml(Stream stream, ParserContext parserContext, Object parent, Boolean closeStream)
at System.Windows.Application.LoadBamlStreamWithSyncInfo(Stream stream, ParserContext pc)
at System.Windows.Application.DoStartup()
at System.Windows.Application.<.ctor>b__1_0(Object unused)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.DispatcherOperation.InvokeImpl()
at MS.Internal.CulturePreservingExecutionContext.CallbackWrapper(Object obj)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
at MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
at System.Windows.Threading.DispatcherOperation.Invoke()
at System.Windows.Threading.Dispatcher.ProcessQueue()
at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
at System.Windows.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at WpfApp1.Program.Main()
```

```
Unhandled exception. System.TypeInitializationException: The type initializer for 'MS.Internal.TraceRoutedEvent' threw an exception.
---> System.InvalidOperationException: Nullable object must have a value.
at System.Nullable`1.get_Value()
at MS.Internal.AvTrace.IsWpfTracingEnabledInRegistry()
at MS.Internal.AvTrace.ShouldCreateTraceSources()
at MS.Internal.AvTrace.Initialize()
at MS.Internal.TraceRoutedEvent..cctor()
--- End of inner exception stack trace ---
at MS.Internal.TraceRoutedEvent.get_IsEnabled()
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.FrameworkElement.OnRenderSizeChanged(SizeChangedInfo sizeInfo)
at System.Windows.ContextLayoutManager.fireSizeChangedEvents()
at System.Windows.ContextLayoutManager.UpdateLayout()
at System.Windows.Interop.HwndSource.SetLayoutSize()
at System.Windows.Interop.HwndSource.set_RootVisualInternal(Visual value)
at System.Windows.Window.SetRootVisualAndUpdateSTC()
at System.Windows.Window.SetupInitialState(Double requestedTop, Double requestedLeft, Double requestedWidth, Double requestedHeight)
at System.Windows.Window.CreateSourceWindow(Boolean duringShow)
at System.Windows.Window.ShowHelper(Object booleanBox)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.DispatcherOperation.InvokeImpl()
at MS.Internal.CulturePreservingExecutionContext.CallbackWrapper(Object obj)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
at MS.Internal.CulturePreservingExecutionContext.Run(CulturePreservingExecutionContext executionContext, ContextCallback callback, Object state)
at System.Windows.Threading.DispatcherOperation.Invoke()
at System.Windows.Threading.Dispatcher.ProcessQueue()
at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
at System.Windows.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at WpfApp1.Program.Main()
```

### Reproduction Steps

We are experiencing this as a very rare occurrence on some machines. I have created a [minimal reproduction case that forces the issue to happen more frequently](https://github.com/ssrmm/AvTraceCrashRepro). The important parts are in `Program.cs`.

Simply starting this application a few times produces the crash pretty easily on my machine.

### Expected behavior

Preferred:
Crash is avoided by making `AvTrace` properly thread-safe.

Alternatively:
Requirement of calling `PresentationTraceSources.Refresh()` on the UI thread is documented and ideally enforced by always throwing an exception when called on the wrong thread.

### Actual behavior

A crash occurs very rarely and only on specific systems, making this problem hard to find during testing.

### Regression?

Looking at the Git history, a regression seems unlikely. The code in question seems to have been unchanged for a long time.

### Known Workarounds

Perform calls to `PresentationTraceSources.Refresh()` on the UI thread only.

### Impact

Rare application startup crash that may only happen on customer machines and is difficult to find during development/testing.

### Configuration

.NET 6/.NET 8 x64
Windows 10/Windows 11

### Other information

_No response_