fix: NullReferenceException in MessageService.ShowMessageBarAsync

[JamesNK]

🐛 Bug Report

I see about 30 NullReferenceException errors per week from MessageService.ShowMessageBarAsync in my app.

I have a component that references IMessageService and can call ShowMessageBarAsync. But this sometimes errors. I think the problem is ShowMessageBarAsync assumes that OnMessageItemsUpdatedAsync has been subscribed to:

fluentui-blazor/src/Core/Components/MessageBar/Services/MessageService.cs

Line 266 in 0625bb0

await OnMessageItemsUpdatedAsync!.Invoke();

This happens in FluentMessageBarProvider.OnInitialize:

fluentui-blazor/src/Core/Components/MessageBar/FluentMessageBarProvider.razor.cs

Line 93 in 0625bb0

MessageService.OnMessageItemsUpdatedAsync += OnMessageItemsUpdatedHandlerAsync;

💻 Repro or Code Sample

🤔 Expected Behavior

No error.

😯 Current Behavior

There is a race between OnInitialize in the two controls causing NullReferenceExceptions.

💁 Possible Solution

MessageService doesn't call the event if it isn't registered?

Also, isn't AllMessages not thread safe? The lock around it just locks while accessing the property. A copy isn't being returned so a message could be added to it while the message bar provider is enumerating the collection, causing an error. Either a copy needs to be made here, or a copy is made each time the message list is modifed.

🔦 Context

🌍 Your Environment

• OS & Device: [e.g. MacOS, iOS, Windows, Linux] on [iPhone 7, PC]
• Browser [e.g. Microsoft Edge, Google Chrome, Apple Safari, Mozilla FireFox]
• .NET and Fluent UI Blazor library Version [e.g. 8.0.2 and 4.4.1]