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

@@ -36,23 +36,26 @@ namespace StardewModdingAPI.Framework.Reflection
         public TInterface CreateProxy<TInterface>(object instance, string sourceModID, string targetModID)
             where TInterface : class
         {
-            // validate
+            lock (this.Builders)
-            if (instance == null)
-                throw new InvalidOperationException("Can't proxy access to a null API.");
-            if (!typeof(TInterface).IsInterface)
-                throw new InvalidOperationException("The proxy type must be an interface, not a class.");
-
-            // get proxy type
-            Type targetType = instance.GetType();
-            string proxyTypeName = $"StardewModdingAPI.Proxies.From<{sourceModID}_{typeof(TInterface).FullName}>_To<{targetModID}_{targetType.FullName}>";
-            if (!this.Builders.TryGetValue(proxyTypeName, out InterfaceProxyBuilder builder))
             {
-                builder = new InterfaceProxyBuilder(proxyTypeName, this.ModuleBuilder, typeof(TInterface), targetType);
+                // validate
-                this.Builders[proxyTypeName] = builder;
+                if (instance == null)
-            }
+                    throw new InvalidOperationException("Can't proxy access to a null API.");
+                if (!typeof(TInterface).IsInterface)
+                    throw new InvalidOperationException("The proxy type must be an interface, not a class.");
 
-            // create instance
+                // get proxy type
-            return (TInterface)builder.CreateInstance(instance);
+                Type targetType = instance.GetType();
+                string proxyTypeName = $"StardewModdingAPI.Proxies.From<{sourceModID}_{typeof(TInterface).FullName}>_To<{targetModID}_{targetType.FullName}>";
+                if (!this.Builders.TryGetValue(proxyTypeName, out InterfaceProxyBuilder builder))
+                {
+                    builder = new InterfaceProxyBuilder(proxyTypeName, this.ModuleBuilder, typeof(TInterface), targetType);
+                    this.Builders[proxyTypeName] = builder;
+                }
+
+                // create instance
+                return (TInterface)builder.CreateInstance(instance);
+            }
         }
     }
 }