NullReferenceException in Dispatcher.VerifyAccess() when using Avalonia in embedded WPF process (no Application.Start())

Problem

When using Avalonia 12.x inside an existing WPF process (e.g. Revit/Dynamo) without calling Application.Start(), setting any property on Window (e.g. Title) throws a NullReferenceException:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Avalonia.AvaloniaObject.VerifyAccess()
   at Avalonia.AvaloniaObject.SetValue[T](StyledProperty`1 property, T value, BindingPriority priority)
   at Avalonia.Controls.Window.set_Title(String value)

Root Cause

The call chain:

1. Window.set_Title("...") → calls AvaloniaObject.SetValue()
2. SetValue() → calls VerifyAccess() for thread validation
3. VerifyAccess() → calls Dispatcher?.VerifyAccess()
4. Dispatcher.VerifyAccess() → accesses internal field _dispatcherImpl (or _jobRunner)
5. If the message pump (Win32 GetMessage loop) is not started → _dispatcherImpl = null → NullReferenceException

Why the message pump is not started

In a standard Avalonia app, Application.Start(ClassicDesktopLifetime) starts the message pump. In an embedded scenario (inside Revit/Dynamo), the message pump is already running via WPF. Avalonia cannot start a second message pump — it conflicts with the main one.

However, Application.Initialize() and AppBuilder.SetupWithoutStarting() correctly create Application.Current and register the platform (UsePlatformDetect), but the Dispatcher remains in a partially-initialized state.

Environment

• Avalonia 12.1.2 (NuGet)
• .NET 8.0
• Revit 2027 + Dynamo (WPF process)
• Windows 10/11

Proposed Fix

Option 1: Null-check in Dispatcher.VerifyAccess() (primary)

File: src/Avalonia.Base/Threading/Dispatcher.cs

// BEFORE:
public void VerifyAccess()
{
    _dispatcherImpl.VerifyAccess();
}

// AFTER:
public void VerifyAccess()
{
    _dispatcherImpl?.VerifyAccess();
}

Rationale: If _dispatcherImpl is null, the Dispatcher is not fully initialized (message pump not started). In this case the thread check is meaningless — skip it.

Option 2: Null-check in AvaloniaObject.VerifyAccess() (alternative)

File: src/Avalonia.Base/AvaloniaObject.cs

// BEFORE:
protected void VerifyAccess()
{
    Dispatcher?.VerifyAccess();
}

// AFTER:
protected void VerifyAccess()
{
    try { Dispatcher?.VerifyAccess(); }
    catch (NullReferenceException) { }
}

Option 3: Static flag to disable thread checks (extended)

Add a static property to Dispatcher:

public static bool SkipThreadChecks { get; set; }

public void VerifyAccess()
{
    if (SkipThreadChecks) return;
    _dispatcherImpl?.VerifyAccess();
}

Allows the embedder to disable checks:

Avalonia.Threading.Dispatcher.SkipThreadChecks = true;

How to Verify

Repro scenario (PythonNet3 + Revit)

from Avalonia import Application, AppBuilder
from Avalonia.Controls import Window

# Initialize WITHOUT Application.Start()
builder = AppBuilder.Configure(Application()) \
    .UsePlatformDetect()
builder.SetupWithoutStarting()

# This should NOT crash:
window = Window()
window.Title = "Test"        # This line crashes without the fix
window.Width = 400
window.Height = 300
print("OK: Window properties set without NullReferenceException")

Expected result

• Before fix: NullReferenceException at window.Title = "Test"
• After fix: Properties are set without errors

Impact

Minimal. Null-check in VerifyAccess() is a safe change. If _dispatcherImpl is null → check is skipped → no side effects. Existing apps (where message pump is running) continue to work as before.

Related Files

Priority

High. Without this fix, Avalonia cannot be used in embedded scenarios (inside Revit, Dynamo, AutoCAD, and other WPF applications).