Fix BindingEvaluator.ClearDataContext

[MrJul]

What does the pull request do?

This PR fixes BindingEvaluator.ClearDataContext, which wasn't really clearing the data context.

What is the current behavior?

The data context is set to the binding evaluator itself, causing exceptions.
The issue was introduced in #18405.

What is the updated/expected behavior with this PR?

The data context is correctly set to null.

Fixed issues

• Fixes ComboBox binding throws exception: Unable to cast object of type 'Avalonia.Controls.Utils.BindingEvaluator`1[System.Object]' to #18822

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0056801-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

It might not matter here, but there two different ways to clear data context - set to null, preventing inheriting value, and calling AvaloniaObject.ClearValue to reset value store entry.

[MrJul]

It might not matter here, but there two different ways to clear data context - set to null, preventing inheriting value, and calling AvaloniaObject.ClearValue to reset value store entry.

Indeed in this case it doesn't really matter since the object isn't attached to any tree. Setting it to null is probably the fastest in this case, since the BindingEvaluator may be reused later: the local values frame will be kept.

That said, I haven't measured it, and it's probably a micro optimization where it doesn't really matter: the vast majority of time spent would be in evaluating the various items' bindings, not clearing the data context once.

I'm keeping null here, we can revisit later if needed.