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);
-        currentTPSLimit = d;
-        instance.tpsLimiter();
+        if (instance != null) {
+            currentTPSLimit = d;
+            instance.tpsLimiter();
+        }
     }
 
 }