not sure how I missed enabling nullable here

Author: SuuperW

src/BizHawk.Client.Common/FileStreamWithTemp.cs

@@ -1,4 +1,6 @@
-using System.Diagnostics;
+#nullable enable
+
+using System.Diagnostics;
 using System.IO;
 
 using BizHawk.Common.StringExtensions;
@@ -7,7 +9,7 @@ namespace BizHawk.Client.Common
 {
 	internal class FileStreamWithTemp : IDisposable
 	{
-		public FileStream Stream;
+		public FileStream? Stream { get; private set; }
 		public string FinalPath;
 		public string TempPath;
 

src/BizHawk.Client.Common/FrameworkZipWriter.cs

@@ -41,7 +41,7 @@ public static FileWriteResult<FrameworkZipWriter> Create(string path, int compre
 
 			FrameworkZipWriter ret = new(compressionLevel);
 			ret._fs = fs.Value!;
-			ret._archive = new(ret._fs.Stream, ZipArchiveMode.Create, leaveOpen: true);
+			ret._archive = new(ret._fs.Stream!, ZipArchiveMode.Create, leaveOpen: true);
 
 			return fs.Convert(ret);
 		}