NullReferenceException thrown in WinRT.Runtime.dll calling GattCharacteristic.WriteValueAsync in .NET 5

[AndyCW]

Describe the bug
Using the Windows.Devices.Bluetooth apis to write to a GattCharacteristic of a Bluetooth LE Service, using the Windows.Devices.Bluetooth.GenericAttributeProfile.GattCharacteristic.WriteValueAsync API. Code works in .NET Framework 4.7.2, same code fails in .NET 5 with NullReferenceException thrown

To Reproduce

1. Create a .NET 5 Console app
2. Set TargetFramework in the csproj file to net5-windows10.0.19041.0
3. Use
   Program.zip
4. Modify Guids in code to match your Bluetooth LE device service and Gatt characteristic
5. Run - observe exception
6. Repeat, but with .NET Framework 4.7.2 console app, adding Windows SDK reference using the Microsoft.Windows.SDK.Contracts NuGet package. Observe that that works.

Expected behavior
.Net 5 app should run to completion successfully

Version Info
Visual Studio 2019, .Net 5, net5-windows10.0.19041.0

Additional context

[angelazhangmsft]

This looks like a duplicate of #755. The fix for that issue will be available in a future .NET SDK update.