diff --git a/src/src/main/java/com/my/graphiteDigesterBg/resource/ResHeatingTubeRackSlotManager.java b/src/src/main/java/com/my/graphiteDigesterBg/resource/ResHeatingTubeRackSlotManager.java
index 0c659f5..7f1b68e 100644
--- a/src/src/main/java/com/my/graphiteDigesterBg/resource/ResHeatingTubeRackSlotManager.java
+++ b/src/src/main/java/com/my/graphiteDigesterBg/resource/ResHeatingTubeRackSlotManager.java
@@ -46,6 +46,9 @@ public class ResHeatingTubeRackSlotManager extends DiResourceManagerBase {
 
     // stop temperature refresh
     public void stopTemperatureRefresh() {
+        if ( null == this.temperatureRefreshTimer ) {
+            return ;
+        }
         this.temperatureRefreshTimer.cancel();
     }