Fix NPE

BauSystem_Main/src/de/steamwar/bausystem/commands/CommandTPSLimiter.java

@@ -167,8 +167,10 @@ public class CommandTPSLimiter extends SWCommand {
     public static void setTPS(double d) {
         if (d < 0.5) d = 0.5;
         if (d > (TPSUtils.isWarpAllowed() ? 40 : 20)) d = (TPSUtils.isWarpAllowed() ? 40 : 20);
+        if (instance != null) {
             currentTPSLimit = d;
             instance.tpsLimiter();
         }
+    }
 
 }