Safe calorie diff.

contentScript.js

@@ -373,6 +373,10 @@ if (typeof Overlay === "undefined") {
     }
 
     calorieDiff() {
+      if (this.workoutStatus == null) {
+        return 0;
+      }
+
       const {minutes, seconds, calories} = this.workoutStatus;
 
       const expected = this.expectedCalories(minutes, seconds);