mine lag fix

src/engine/gameManager/SimulationManager.java

@@ -252,7 +252,7 @@ public enum SimulationManager {
                 continue;
             }
 
-            if (mine.isActive && (LocalDateTime.now().isAfter(closeTime) || LocalDateTime.now().isBefore(openTime))) {
+            if (mine.isActive && mine.lastClaimer != null && (LocalDateTime.now().isAfter(closeTime) || LocalDateTime.now().isBefore(openTime))) {
                 mineWindowClose(mine);
             }
         }