OMA-74: fix a deadly missing line (and make the controller robust)

src/main/java/org/olat/commons/calendar/ui/WeeklyCalendarController.java

@@ -223,7 +223,7 @@ public class WeeklyCalendarController extends BasicController implements Activat
 		weeklyCalendar.addListener(this);
 
 	  // subscription, see OLAT-3861
-		if (!isGuest) {
+		if (!isGuest && !calendarWrappers.isEmpty()) {
 			SubscriptionProvider provider = new SubscriptionProviderImpl(caller, calendarWrappers.get(0));
 			subsContext = provider.getSubscriptionContext();
 			// if sc is null, then no subscription is desired

src/main/java/org/olat/course/nodes/cal/CourseCalendars.java

@@ -121,6 +121,7 @@ public class CourseCalendars {
 		ICourse course = CourseFactory.loadCourse(ores);
 		CourseLinkProviderController clpc = new CourseLinkProviderController(course, ureq, wControl);
 		courseKalendarWrapper.setLinkProvider(clpc);
+		calendars.add(courseKalendarWrapper);
 		
 		Identity identity = ureq.getIdentity();
 		CourseGroupManager cgm = course.getCourseEnvironment().getCourseGroupManager();