bug fix null pointer exception in enrollment table view

src/main/java/de/thdeg/grademanager/model/EnrollmentProperty.java

@@ -19,7 +19,7 @@ public class EnrollmentProperty {
     /**
      * The variable which stores the grade value.
      */
-    private SimpleDoubleProperty gradeProp;
+    private SimpleStringProperty gradeProp;
 
 
     /**
@@ -31,7 +31,11 @@ public class EnrollmentProperty {
      */
     public EnrollmentProperty(Enrollment enrollment) {
         courseProp = new SimpleStringProperty(enrollment.getCourse().getName());
-        gradeProp = new SimpleDoubleProperty(enrollment.getGrade());
+        if (enrollment.getGrade() == null){
+            gradeProp = new SimpleStringProperty("-");
+        } else {
+            gradeProp = new SimpleStringProperty(enrollment.getGrade().toString());
+        }
     }
 
     /**
@@ -52,7 +56,7 @@ public class EnrollmentProperty {
      *
      * @return the value of the gradeProp-Object.
      */
-    public double getGradeProp() {
+    public String getGradeProp() {
         return gradeProp.get();
     }