fix coursesOfStudy = null when switching between screens

.idea/workspace.xml

@@ -4,22 +4,9 @@
     <option name="autoReloadType" value="SELECTIVE" />
   </component>
   <component name="ChangeListManager">
-    <list default="true" id="ce56effb-683b-43e9-9335-7e3aa5d26c29" name="Changes" comment="big update:&#10;-add toString() for Course, Student and CoursesOfStudy&#10;-fix null-pointer-exception in detailControllers + SwitchWindowHelper&#10;-add CoursesOfStudy Column in Student&#10;-add find-Methods to JpaService&#10;-connect Gui to DB">
+    <list default="true" id="ce56effb-683b-43e9-9335-7e3aa5d26c29" name="Changes" comment="">
       <change beforePath="$PROJECT_DIR$/.idea/workspace.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/workspace.xml" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/JpaService.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/JpaService.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/CourseModificationController.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/CourseModificationController.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/CoursesOfStudyDetailsController.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/CoursesOfStudyDetailsController.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/CoursesOfStudyModificationController.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/CoursesOfStudyModificationController.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/Main.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/Main.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/MainController.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/MainController.java" afterDir="false" />
       <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/StudentDetailsController.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/StudentDetailsController.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/StudentModificationController.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/StudentModificationController.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/SwitchWindowHelper.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/gui/SwitchWindowHelper.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/model/Course.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/model/Course.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/model/CoursesOfStudy.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/model/CoursesOfStudy.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/model/Student.java" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/java/de/thdeg/grademanager/model/Student.java" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/resources/META-INF/persistence.xml" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/resources/META-INF/persistence.xml" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/src/main/resources/de/thdeg/grademanager/gui/student-details.fxml" beforeDir="false" afterPath="$PROJECT_DIR$/src/main/resources/de/thdeg/grademanager/gui/student-details.fxml" afterDir="false" />
     </list>
     <option name="SHOW_DIALOG" value="false" />
     <option name="HIGHLIGHT_CONFLICTS" value="true" />
@@ -97,7 +84,14 @@
       <option name="project" value="LOCAL" />
       <updated>1655399588913</updated>
     </task>
-    <option name="localTasksCounter" value="2" />
+    <task id="LOCAL-00002" summary="big update:&#10;-add toString() for Course, Student and CoursesOfStudy&#10;-fix null-pointer-exception in detailControllers + SwitchWindowHelper&#10;-add CoursesOfStudy Column in Student&#10;-add find-Methods to JpaService&#10;-connect Gui to DB">
+      <created>1655458541861</created>
+      <option name="number" value="00002" />
+      <option name="presentableId" value="LOCAL-00002" />
+      <option name="project" value="LOCAL" />
+      <updated>1655458541861</updated>
+    </task>
+    <option name="localTasksCounter" value="3" />
     <servers />
   </component>
   <component name="Vcs.Log.Tabs.Properties">

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

@@ -66,10 +66,12 @@ public class StudentDetailsController {
 
     @FXML
     protected void assignCoursesOfStudy(ActionEvent event) {
-        student.setCoursesOfStudy(coursesOfStudyComboBox.getValue());
-        JpaService jpaService = JpaService.getInstance();
-        jpaService.runInTransaction(entityManager -> {entityManager.merge(student); return null;});
-        initialize();
+        if (coursesOfStudyComboBox.getValue() != null){
+            student.setCoursesOfStudy(coursesOfStudyComboBox.getValue());
+            JpaService jpaService = JpaService.getInstance();
+            jpaService.runInTransaction(entityManager -> {entityManager.merge(student); return null;});
+            initialize();
+        }
     }
 
     @FXML