BindableObject System.NullReferenceException when bindings queue gets messed up

[taublast]

Description of Change

Added a check for null.

Issues Fixed

What I am meeting often in a multi-threaded situation:

System.NullReferenceException: Object reference not set to an instance of an object.

03-29 10:10:25.660 I/DOTNET  ( 4189): System.NullReferenceException: Object reference not set to an instance of an object.
03-29 10:10:25.661 I/DOTNET  ( 4189):    at Microsoft.Maui.Controls.BindableObject.SetValueCore(BindableProperty property, Object value, SetValueFlags attributes, SetValuePrivateFlags privateAttributes, SetterSpecificity specificity) in D:\a\_work\1\s\src\Controls\src\Core\BindableObject.cs:line 577
03-29 10:10:25.661 I/DOTNET  ( 4189):    at Microsoft.Maui.Controls.BindableObject.SetValue(BindableProperty property, Object value) in D:\a\_work\1\s\src\Controls\src\Core\BindableObject.cs:line 474

This leads to bindings to stop working forever for this specific BindableObject.

Proposing that the backend should never crash whatever the client is doing.

[dotnet-policy-service]

Hey there @taublast! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[rmarinho]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

[rmarinho]

Hey @taublast do you think you can add a unit test? thanks

[taublast]

Hey @taublast do you think you can add a unit test? thanks

@rmarinho sure, will do.

[taublast]

Still on it, very hard to catch and reproduce, can't get a 100% repro yet.

Debugging that test catched another 100%-related problem, I was getting it since MAUI came out, but it was very hard to reproduce. Should we fix it too?

My guess the solution is to lock execution of GetOrCreateContext since its parallel execution can access the non-concurrent Dictionary inside. Another solution could be to use "ConcurrentDictionary".

[taublast]

For me this one catches either crash A or crash B from the above post:

[Fact]
	public async Task GetSetValueMultithreaded()
	{
		var value = "text";
		int numberOfThreads = 100;
		var random = new Random();
		var exceptions = new ConcurrentBag<Exception>();
		bool get = false;

		for (int i = 0; i < numberOfThreads; i++)
		{
			var newMock = new MockBindable();
			Parallel.For(0, numberOfThreads, async (i) =>
			{
				try
				{
					if (random.Next(0, 1) == 1)
					{
						var read = newMock.GetValue(MockBindable.TextProperty);
					}
					else
					{
						newMock.SetValue(MockBindable.TextProperty, value);
					}
				}
				catch (Exception ex)
				{
					exceptions.Add(ex);
				}
			});
		}

		if (exceptions.Count > 0)
		{
			throw new AggregateException(exceptions);
		}
	}

[rmarinho]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).