fix crash when updating blacklist of ReShade is installed

Author: Pathoschild

docs/release-notes.md

@@ -4,6 +4,7 @@
 ## Upcoming release
 * For players:
   * Added a friendly error message when the game fails to launch with a `NoSuitableGraphicsDeviceException`.
+  * Fixed crash when SMAPI tries to update the mod blacklist if ReShade is installed.
 
 ## 4.3.1
 Released 13 July 2025 for Stardew Valley 1.6.14 or later. See [release highlights](https://www.patreon.com/posts/133992196).

src/SMAPI.Web/Startup.cs

@@ -49,7 +49,7 @@ internal class Startup
     private const string BlacklistFileName = "SMAPI.blacklist.json";
 
     /// <summary>The MD5 hash for the 'malicious mods' blacklist file.</summary>
-    private Lazy<string?> BlacklistMd5Hash;
+    private readonly Lazy<string> BlacklistMd5Hash;
 
 
     /*********