OO-4748: a group configured for enrollment can be deleted

src/main/java/org/olat/course/nodes/en/ENEditGroupAreaFormController.java

@@ -187,12 +187,13 @@ class ENEditGroupAreaFormController extends FormBasicController implements Gener
 		List<EnrollmentRow> enrollmentRows = enrollmentManager.getEnrollments(getIdentity(), groupKeys, null, 256);
 		Map<Long,EnrollmentRow> enrollmentMap = enrollmentRows.stream().collect(Collectors.toMap(EnrollmentRow::getKey, g -> g, (u, v) -> u));
 
-		easyGroupTableRows = new ArrayList<ENEditGroupTableContentRow>();
+		easyGroupTableRows = new ArrayList<>();
 		for (Long groupKey : groupKeys) {
 			BusinessGroup group = groupMap.get(groupKey);
-			EnrollmentRow enrollment = enrollmentMap.get(groupKey);
-			
-			easyGroupTableRows.add(new ENEditGroupTableContentRow(group, enrollment));
+			if(group != null) {
+				EnrollmentRow enrollment = enrollmentMap.get(groupKey);
+				easyGroupTableRows.add(new ENEditGroupTableContentRow(group, enrollment));
+			}
 		}
 
 		easyGroupTableModel.setObjects(easyGroupTableRows);