enabled some client side command interpretation.

src/main/java/carpet/CarpetServer.java

@@ -119,6 +119,7 @@ public class CarpetServer // static for now - easier to handle all around the co
         settingsManager.detachServer();
         LoggerRegistry.stopLoggers();
         extensions.forEach(e -> e.onServerClosed(server));
+        minecraft_server = null;
     }
 
     public static void registerExtensionLoggers()