The GetDocument() is returning a null value after which it's going to the Value cannot be null. (Parameter 'document') exception

[JigyashaOT]

Hi,
I've been trying to use this repo to migrate my Xamarin project to MAUI, but after these 2 lines of code it is going to exception. Any idea why?
var doc = _sourceUpdater.Project.GetRoslynProject().GetDocument(diagnostic.Location.SourceTree)!;
var updatedSolution = await TryFixDiagnosticAsync(diagnostic, doc, token).ConfigureAwait(false);