Scheduling Assistant: Build Schedule

- fixed an null error when using as a guest and there is a course with Linked Classes distribution(s)

JavaSource/org/unitime/timetable/onlinesectioning/model/XDistribution.java

@@ -98,6 +98,7 @@ public boolean hasSection(Long sectionId) {
     }
 
     public boolean isViolated(XStudent student, OnlineSectioningServer server) {
+    	if (student == null) return false;
     	int nrMatch = 0, nrMismatch = 0;
     	for (Long offeringId: getOfferingIds()) {
     		XOffering offering = server.getOffering(offeringId);