small bugfix in modification option

src/main/java/de/thdeg/grademanager/gui/CoursesOfStudyDetailsController.java

@@ -84,9 +84,11 @@ public class CoursesOfStudyDetailsController {
     protected Button saveButton;
     @FXML
     protected void onSaveButtonClick(ActionEvent event) throws IOException {
-        if (!name.getText().isBlank() && durationComboBox.getValue() != null && !fieldOfStudy.getText().isBlank()) {
+        if (!name.getText().isBlank() && !fieldOfStudy.getText().isBlank()) {
             coursesOfStudy.setName(name.getText());
-            coursesOfStudy.setDuration(durationComboBox.getValue().getSemester());
+            if(durationComboBox.getValue() != null){
+                coursesOfStudy.setDuration(durationComboBox.getValue().getSemester());
+            }
             coursesOfStudy.setFieldOfStudy(fieldOfStudy.getText());
 
             JpaService jpaService = JpaService.getInstance();