Fix for #1568 when RaiseAsync with a void Delegate returns null results in System.NullReferenceException

[Keugon]

Hi this is my best guess on improving information on issue when try to use the RaiseAsync() with a void Delegate with a more informativ Exception.

this is my first ever pullrequest and iam pretty sure i done many mistakes
#1568

Help Wanted here

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[Keugon]

is there more that i need to do so this can be considered?

[kzu]

void-returning async methods are a huge red flag in .NET. They lead to unobserved exceptions that can tear down a process. not sure we want to encourage that sort of design... 🤔

[kzu]

I'll still merge it because it's not introducing a breaking change from something that previously worked (nobody would set up this previously), so it's new behavior. Although there is a slight chance that someone was relying on the previous (unrelated) exception to catch this scenario (think: a pseudo-verification by side-effect), that would have been quite unlikely, I'd say.

[Keugon]

@kzu do i need to do anything here, iam still not very familiar with github and opensource repos, i just work with a local gitlab on company repos.

If i need to take action here please tell me