Race condition when connecting to DicomService quickly after start up

[CeeJayCee]

Describe the bug

We have seen an exception being thrown when connecting to a DicomServer quickly after it is started.

In our case, we are starting a DicomServer that implements DicomService, IDicomServiceProvider, IDicomCStoreProvider and IDicomCEchoProvider.

Immediately after creation we use echoscu.exe to check the service is up. Frequently we get the following exception:

Exception listening for DICOM services, NullReferenceException {TargetSite=T CreateScp(FellowOakDicom.Network.INetworkStream), Message="Object reference not set to an instance of an object.", Data=[], InnerException=null, HelpLink=null, Source="fo-dicom.core", HResult=-2147467261, StackTrace="   at FellowOakDicom.Network.DicomServer`1.CreateScp(INetworkStream stream)\n   at FellowOakDicom.Network.DicomServer`1.ListenForConnectionsAsync()"}

Our debugging has shown this is caused by DicomService (line 247) trying to access ServiceScope.ServiceProvider. ServiceScope has not yet been set by DicomServerFactory::Create() before StartAsync() is called on the server.

To Reproduce
As above.

Expected behavior
An exception should not be thrown.

Screenshots or test DICOM files
If applicable, add screenshots or test DICOM files to help explain your problem.

Environment
Fellow Oak DICOM version: 5.0.3
OS: Docker container running mcr.microsoft.com/dotnet/aspnet:6.0-alpine
Platform: .NET 6.0

[CeeJayCee]

I have proposed a fix in #1399