Only revert to frozen data on server shutdown on client. Should fix race condition exposed by #4904

src/main/java/net/minecraftforge/fml/client/FMLClientHandler.java

@@ -705,6 +705,8 @@ public class FMLClientHandler implements IFMLSidedHandler
         {
 //            ObfuscationReflectionHelper.setPrivateValue(MinecraftServer.class, server, true, "field_71296"+"_Q","serverIs"+"Running");
         }
+
+        GameData.revertToFrozen();
     }
 
     @Override

src/main/java/net/minecraftforge/fml/common/Loader.java

@@ -864,7 +864,6 @@ public class Loader
 
     public void serverStopped()
     {
-        GameData.revertToFrozen();
         modController.distributeStateMessage(LoaderState.SERVER_STOPPED);
         modController.transition(LoaderState.SERVER_STOPPED, true);
         modController.transition(LoaderState.AVAILABLE, true);