System.Reflection.TargetInvocationException: 'Exception has been thrown by the target of an invocation.'

Describe the bug Since updating a project to the latest version of WinUI3 (Reunion 0.5) we receive the following error whenever trying to debug:

System.Reflection.TargetInvocationException
  HResult=0x80131604
  Message=Exception has been thrown by the target of an invocation.
  Source=System.Private.CoreLib
  StackTrace:
   at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean wrapExceptions, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& hasNoDefaultCtor)
   at System.RuntimeType.CreateInstanceDefaultCtorSlow(Boolean publicOnly, Boolean wrapExceptions, Boolean fillCache) in /_/src/coreclr/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs:line 4005
   at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, Boolean wrapExceptions) in /_/src/coreclr/src/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs:line 4062
   at System.Activator.CreateInstance[T]() in /_/src/libraries/System.Private.CoreLib/src/System/Activator.RuntimeType.cs:line 151
   at WinRT.WeakLazy`1.get_Value()
   at Microsoft.UI.Xaml.Application._IApplicationStatics.get_Instance()
   at Microsoft.UI.Xaml.Application.Start(ApplicationInitializationCallback callback)
   at Project.Program.Main(String[] args) in C:\Users\USER\Source\Repos\Project\Project.UI\Project.UI\obj\x64\Debug\net5.0-windows10.0.19041.0\App.g.i.cs:line 26

  This exception was originally thrown at this call stack:
    System.Runtime.InteropServices.Marshal.ThrowExceptionForHR(int) in Marshal.cs
    WinRT.BaseActivationFactory.BaseActivationFactory(string, string)
    Microsoft.UI.Xaml.Application._IApplicationStatics._IApplicationStatics()

Inner Exception 1:
COMException: Class not registered (0x80040154 (REGDB_E_CLASSNOTREG))

Steps to reproduce the bug We used a project created in WinUI3 v3 and followed the official instructions to upgrade to Reunion 0.5. The original project works as expected until Reunion is installed/WinUI3 removed - then we are unable to run the project with the CLASSNOTREG error.

Expected behavior The project will run as before.

Screenshots

Version Info NuGet package version: Microsoft.ProjectReunion-v0.5.0-prerelease Microsoft.ProjectReunion.Foundation-v0.5.0-prerelease Microsoft.ProjectReunion.WinUI-v0.5.0-prerelease

Windows app type: Win32

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)	Yes
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

[RealTommyKlein]

@Noemata - agreed it’s painful. We plan to remove the wapproj and only need a single project for Desktop apps for Project Reunion’s 0.8 release.

[github-actions[bot]]

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