Repro steps:

Create a new git repository:

mkdir folder
cd folder
git init

Create a repository instance and try to list commits.

var repo = new Repository(@"C:\dev\folder\.git");
foreach (var c in repo.Commits) {
    Console.WriteLine(c.Id);
}

Expected:

Empty commit collection

Actual:

Null reference exception

I actually dug a little deeper into why it throws. It seems that this line throws (CommitCollection.StartingAt):

return new CommitCollection(repo) { sortOptions = sortOptions, pushedSha =   reference.ResolveToDirectReference().Target.Sha };

ResolveToDirectReference() returns null since the reference is the head which doesn't point to any commits by default.