fix concurrency issue in interface proxying

docs/release-notes.md

@@ -10,6 +10,7 @@
 ## Upcoming release
 * For players:
   * Added error message if you manually install the wrong SMAPI bitness (e.g. 32-bit SMAPI with 64-bit game).
+  * Fixed intermittent error if a mod fetches mod-provided APIs asynchronously.
 
 ## 3.11.0
 Released 09 July 2021 for Stardew Valley 1.5.4 or later. See [release highlights](https://www.patreon.com/posts/53514295).

src/SMAPI/Framework/Reflection/InterfaceProxyFactory.cs

@@ -35,6 +35,8 @@ namespace StardewModdingAPI.Framework.Reflection
         /// <param name="targetModID">The unique ID of the mod providing the API.</param>
         public TInterface CreateProxy<TInterface>(object instance, string sourceModID, string targetModID)
             where TInterface : class
+        {
+            lock (this.Builders)
             {
                 // validate
                 if (instance == null)
@@ -56,3 +58,4 @@ namespace StardewModdingAPI.Framework.Reflection
             }
         }
     }
+}