Fix ServerChatEvent thread not using correct class loader. (#10591)

src/main/java/net/minecraftforge/common/ForgeHooks.java

@@ -40,6 +40,7 @@ import com.mojang.serialization.Lifecycle;
 
 import it.unimi.dsi.fastutil.objects.ObjectArrayList;
 import net.minecraft.ResourceLocationException;
+import net.minecraft.Util;
 import net.minecraft.commands.CommandSourceStack;
 import net.minecraft.commands.Commands;
 import net.minecraft.commands.SharedSuggestionProvider;
@@ -420,7 +421,7 @@ public class ForgeHooks
                 return message; // Vanilla should never get here with the patches we use, but let's be safe with dumb mods
 
             return onServerChatSubmittedEvent(sender, getRawText(message), message);
-        });
+        }, Util.backgroundExecutor());
     }
 
     static final Pattern URL_PATTERN = Pattern.compile(