Added check to see if any period was found

LangsteZomerPeriode.java

@@ -5,14 +5,21 @@ public class LangsteZomerPeriode extends Grootheid{
     
     //constructor
     public LangsteZomerPeriode(Measurement measurement1, ArrayList<Measurement> measurement2){
-    	setName("Zomerse periode");
+        setName("Zomerse periode");
         updatePeriod(measurement2);
     }
 
     public void updatePeriod(ArrayList<Measurement> measurement2){
         createList(measurement2);
         int[] index = StatisticsCalculator.langsteZomersePeriode(list);
-        setPeriod(Calculator.timeStampToPeriode( measurement2.get(index[0]).getDateStamp(), measurement2.get(index[1]).getDateStamp()));
+        if(index[0]==index[1] && index[0]==0)
+        {
+            setCustom("-");
+        }
+        else
+        {
+            setPeriod(Calculator.timeStampToPeriode( measurement2.get(index[0]).getDateStamp(), measurement2.get(index[1]).getDateStamp()));
+        }
     }
     
     public void displayGraph(){}