MessagePackCodeFixProvider crash on code fixit

[gfs]

Bug description

MessagePackCodeFixProvider crashes when uses code fix it to insert keys on an inherited class if the base class hasn't yet been annotated and is in a separate file.

Repro steps

Have MessagePackAnalyzers installed.

Put this code in one file:

namespace AttackSurfaceAnalyzer.Objects
{
    public abstract class CollectObject
    {
        public abstract string Identity { get; }
    }
}

Put this code in a separate file (it doesn't crash if they are in the same file):

using MessagePack;

namespace AttackSurfaceAnalyzer.Objects
{
    [MessagePackObject]
    public class CertificateObject : CollectObject
    {
        public string StoreLocation { get; set; }
        public string StoreName { get; set; }
        public string CertificateHashString { get; set; }
        public string Subject { get; set; }
        public string Pkcs7 { get; set; }

        public override string Identity
        {
            get
            {
                return $"{StoreLocation}{StoreName}{CertificateHashString}";
            }
        }
    }
}

Right click to perform the fix-it to add the keys on any of the fields in CertificateObject and the extension will crash.

Expected behavior

Add keys appropriately.

Actual behavior

What happened instead of what you expected.

• Version used: 2.1.90
• Runtime: .NET Core 3.1

Additional context

Add any other context about the problem here.

[github-actions]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[gfs]

Bump.