[BUG] Delegate proxies crash with NRE when user delegate is null (only if rewrite to function pointers)

[maxkatz6]

Description

This is a weird issue, as I wasn't able to reproduce it with a WASM Mono runtime or normal desktop application. But only with NativeAOT-LLVM.
It seems this issue is only reproducible after I replaced managed delegates with function pointers as part of #2615

But I decided to open this issue here instead, as it actually looks like a bug in SkiaSharp code, and exception is somehow swallowed outside of it.

Code

SkiaSharp has many methods accepting callbacks. Like:

public bool InstallPixels (SKImageInfo info, IntPtr pixels, int rowBytes, SKBitmapReleaseDelegate releaseProc, object context)
{
	var cinfo = SKImageInfoNative.FromManaged (ref info);
	var del = releaseProc != null && context != null
		? new SKBitmapReleaseDelegate ((addr, _) => releaseProc (addr, context))
		: releaseProc;
	DelegateProxies.Create (del, out _, out var ctx);
	return SkiaApi.sk_bitmap_install_pixels (Handle, &cinfo, (void*)pixels, (IntPtr)rowBytes, DelegateProxies.SKBitmapReleaseDelegateProxy, (void*)ctx);
}

The problem with this callback, that it passes proxy function to the DllImport even when releaseProc is null.

And from proxy implementation, it will always crash with NRE, as context is an empty pointer.

private static void SKSurfaceReleaseDelegateProxyImplementation (void* address, void* context)
{
	var del = Get<SKSurfaceReleaseDelegate> ((IntPtr)context, out var gch);
	try {
		del.Invoke ((IntPtr)address, null);
	} finally {
		gch.Free ();
	}
}

Expected Behavior

No exception.

Actual Behavior

Exception.

Version of SkiaSharp

3.x (Alpha)

Last Known Good Version of SkiaSharp

Other (Please indicate in the description)

IDE / Editor

Visual Studio (Windows)

Platform / Operating System

Other (Please indicate in the description)

Platform / Operating System Version

Reproduced on WASM (both Mono and NativeAOT)
_

Relevant Log Output

dotnet.native.js:1553 Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.

	put_char	@	dotnet.native.js:1553
	$SystemNative_LogError	@	dotnet.native.wasm:0x14219fc
	$S_P_CoreLib_Internal_Console_Error__Write	@	dotnet.native.wasm:0x6a6783
	$S_P_CoreLib_System_RuntimeExceptionHelpers__FailFast	@	dotnet.native.wasm:0xd0c2d8
	$S_P_CoreLib_System_RuntimeExceptionHelpers__RuntimeFailFast	@	dotnet.native.wasm:0x10b8b5b
	$S_P_CoreLib_System_Runtime_EH__HandleUnhandledException	@	dotnet.native.wasm:0x10b6dfa
	$SkiaSharp_SkiaSharp_DelegateProxies__SKBitmapReleaseDelegateProxyImplementation$F1_Filter	@	dotnet.native.wasm:0xed436a
	$S_P_CoreLib_System_Runtime_EH__CallFilterFunclet	@	dotnet.native.wasm:0x114210f
	$S_P_CoreLib_System_Runtime_EH__DispatchException	@	dotnet.native.wasm:0xd903c
	$S_P_CoreLib_System_Runtime_EH__RhpThrowEx	@	dotnet.native.wasm:0x108ba01
	$S_P_CoreLib_Internal_Runtime_CompilerHelpers_ThrowHelpers__ThrowNullReferenceException	@	dotnet.native.wasm:0x108b96d
	$SkiaSharp_SkiaSharp_DelegateProxies__SKBitmapReleaseDelegateProxyImplementation	@	dotnet.native.wasm:0xed4317
	$SkMakePixelRefWithProc(int, int, unsigned long, void*, void ()(void, void*), void*)::PixelRef::~PixelRef()	@	dotnet.native.wasm:0x185b3e4
	$SkRefCntBase::internal_dispose() const	@	dotnet.native.wasm:0x18765ed
	$SkBitmap::~SkBitmap()	@	dotnet.native.wasm:0x185b791
	$sk_bitmap_destructor	@	dotnet.native.wasm:0x1943b5e
	$SkiaSharp_SkiaSharp_SKBitmap__DisposeNative	@	dotnet.native.wasm:0xdbe66c
	$SkiaSharp_SkiaSharp_SKNativeObject__Dispose	@	dotnet.native.wasm:0xa68216
	$SkiaSharp_SkiaSharp_SKBitmap__Dispose	@	dotnet.native.wasm:0xdbe69d
	$SkiaSharp_SkiaSharp_SKNativeObject__Dispose_0	@	dotnet.native.wasm:0xc74915
	$Avalonia_Skia_Avalonia_Skia_ImmutableBitmap___ctor_3

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[maxkatz6]

Updated description, as I was able to find more specific reason.
This crash doesn't happen on master or any nuget build, but code itself is faulty.