public static string MkisofsPath => Path.Combine(Application.StartupPath, "External", "mkisofs", "mkisofs.exe");

[...]

 using (var archive = RarArchive.OpenArchive(rarPath))
 {
     foreach (var entry in archive.Entries)
     {
         if (
             !entry.IsDirectory && 
             entry.Key.EndsWith("mkisofs.exe", StringComparison.OrdinalIgnoreCase)
             )
         {
             Directory.CreateDirectory(Path.GetDirectoryName(MkisofsPath)!);

             using (var outStream = File.Create(MkisofsPath))
             {
                 // System.NullReferenceException on this line:
                 entry.WriteTo(outStream);
             }

             // 3. Cleanup
             try { File.Delete(rarPath); } catch { }
             return;
         }
     }
 throw new FileNotFoundException("mkisofs.exe not found in archive");