[6.0.100-preview.4.21206.10][main branch] LibGit2Sharp System.NullReferenceException at git_diff_find_similar native method

[jiangzeng01]

Application Name: LibGit2Sharp (https://github.com/libgit2/libgit2sharp)
OS: Windows 10 RS5
CPU: X64
.NET Build Number: 6.0.100-preview.4.21206.10
Affected Application: git-istage global tool app (GlobalToolsNETCore21Part01)
https://github.com/terrajobst/git-istage

Verify Scenarios:
1). Windows 10 RS5 X64 + .NET Core SDK build 6.0.100-preview.4.21206.1 - main branch: Fail
2). Windows 10 RS5 X64 + .NET Core SDK build 6.0.100-preview.3.21202.5 - Preview 3 branch: Pass
3). Windows 10 RS5 X64 + .NET Core SDK build 5.0.104 : Pass

DevDiv bug: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1307298

Repro steps to run

1. Create a .net core console app
2. Add <PackageReference Include="LibGit2Sharp" Version="0.26.2" />
3. In Program.cs

var _repository = new Repository(@".git folder path in the project");
var tipTree = _repository.Head.Tip?.Tree;
var comp = _repository.Diff.Compare<TreeChanges>(tipTree, DiffTargets.Index);  //exception here
Console.WriteLine(comp.Count); //changes count

Expected Result:
Should return the number of staged changes

Actual Result:

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Object.GetType()
   at LibGit2Sharp.Core.NativeMethods.git_diff_find_similar(git_diff* diff, GitDiffFindOptions options)
   at LibGit2Sharp.Core.Proxy.git_diff_find_similar(DiffHandle diff, GitDiffFindOptions options)
   at LibGit2Sharp.Diff.DetectRenames(DiffHandle diffList, CompareOptions compareOptions)
   at LibGit2Sharp.Diff.BuildDiffList(ObjectId oldTreeId, ObjectId newTreeId, TreeComparisonHandleRetriever comparisonHandleRetriever, DiffModifiers diffOptions, IEnumerable`1 paths, ExplicitPathsOptions explicitPathsOptions, CompareOptions compareOptions)
   at LibGit2Sharp.Diff.Compare[T](Tree oldTree, DiffTargets diffTargets, IEnumerable`1 paths, ExplicitPathsOptions explicitPathsOptions, CompareOptions compareOptions)
   at LibGit2Sharp.Diff.Compare[T](Tree oldTree, DiffTargets diffTargets)
   at ConsoleApp1.Program.Main(String[] args) in C:\Users\v-altunc\source\repos\ConsoleApp12\ConsoleApp1\Program.cs:line 14

@dotnet-actwx-bot @dotnet/compat

[jiangzeng01]

@jkoritzinsky @AaronRobinsonMSFT @elinor-fung This issue happened after the issue #50410 was fixed.
Could you please check about this?

[AaronRobinsonMSFT]

@jiangzeng01 Yes, we did observe this issue and I believe @jkoritzinsky has addressed that in our .NET 5 servicing and also recently put in a fix for .NET 6.

[AaronRobinsonMSFT]

See #50735.

[jiangzeng01]

@AaronRobinsonMSFT Thanks! We will verify this with main branch builds.

[jiangzeng01]

@AaronRobinsonMSFT This case is still failing today's latest SDK of main branch: dotnet-sdk-6.0.100-preview.4.21210.1. Could you please take a look?

[AaronRobinsonMSFT]

@jiangzeng01 Thanks. This will be investigated today.