fixes: some null annotation

[workgroupengineering]

What does the pull request do?

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[workgroupengineering]

Can be null?

Avalonia/src/Avalonia.Controls/Application.cs

Lines 101 to 123 in a2ca358

	/// <summary>
	/// Gets the application's focus manager.
	/// </summary>
	/// <value>
	/// The application's focus manager.
	/// </value>
	public IFocusManager FocusManager
	{
	get;
	private set;
	}
	/// <summary>
	/// Gets the application's input manager.
	/// </summary>
	/// <value>
	/// The application's input manager.
	/// </value>
	public InputManager InputManager
	{
	get;
	private set;
	}

[grokys]

Note that C# now supports T? to mean "non-null for value types, nullable for reference types" - it has very similar semantics to [AllowNull] but can be used in a type parameter.

This wasn't supported when a lot of this code was written which is why it wasn't used before.

Regarding the warnings that are due to a C# bug - I'd suggest leaving them as warnings for now - we're hopefully going to be updating our SDK soon which should fix them.

[maxkatz6]

@workgroupengineering we have updated SDK to 5.0, should it be reviewed?

[workgroupengineering]

I am unsure how to correct these warnings.

##[warning]/home/vsts/work/1/s/src/Avalonia.Controls/Application.cs(64,16): warning CS8618: Non-nullable property 'FocusManager' must contain a non-null value when exiting constructor. Consider declaring the property as nullable. [/home/vsts/work/1/s/src/Avalonia.Controls/Avalonia.Controls.csproj]
##[warning]/home/vsts/work/1/s/src/Avalonia.Controls/Application.cs(64,16): warning CS8618: Non-nullable property 'InputManager' must contain a non-null value when exiting constructor. Consider declaring the property as nullable. [/home/vsts/work/1/s/src/Avalonia.Controls/Avalonia.Controls.csproj]

before the RegisterServices method is called they can have a null value. I do not know whether to suppress the warning or declare them as nullable.