An external event created outside an add-in callback runs its handler with no active add-in #134

A Toolkit external event runs its handler as the add-in that uses it, wherever the event is first created.

ExternalEventHandler creates the Revit ExternalEvent inside RevitContext.BeginApiContextScope(), which opens the call depth scope and no add-in data scope.
Revit records the active add-in when ExternalEvent.Create runs, and runs the handler as that add-in.
Outside an add-in callback, such as in a WPF event handler of a modeless window, in a dispatcher callback, or on a thread-pool thread, no add-in is active.
A generated [ExternalEvent] property creates its event on the first access, so the first caller decides the add-in of every later raise.

Steps to reproduce

1. Create a schema with SetVendorId("Contoso"), public read access, and vendor write access.
2. In a session whose add-in has the vendor Contoso, create an AsyncRequestExternalEvent<string> whose handler reads UIApplication.ActiveAddInId and writes an entity of the schema to a new document.
3. Create the event in four places and raise it.

✔️ Expected Behavior

The handler runs as the add-in in every case, and the write succeeds.

❌ Actual Behavior

Environment

Revit 2025, Nice3point.Revit.Toolkit 2025.2.3, Nice3point.TUnit.Revit 2025.1.8 with RevitClient(VendorId = "Contoso"), a RevitApiUiTest.