Fix sort date (move dates with no end date to the back

client/src/stores/courseSessions.ts

@@ -201,7 +201,18 @@ export const useCourseSessionsStore = defineStore("courseSessions", () => {
     allCourseSessions.value?.forEach((cs) => {
       allDueDatesReturn.push(...cs.due_dates);
     });
-    allDueDatesReturn.sort((a, b) => dayjs(a.end).diff(dayjs(b.end)));
+
+    allDueDatesReturn.sort((a, b) => {
+      const dateA = dayjs(a.end);
+      const dateB = dayjs(b.end);
+
+      if (!dateA.isValid() && !dateB.isValid()) return 0; // If both are invalid, they are equal
+      if (!dateA.isValid()) return 1; // If dateA is invalid, it goes after dateB
+      if (!dateB.isValid()) return -1; // If dateB is invalid, it goes after dateA
+
+      return dateA.diff(dateB); // Otherwise, sort by the end date
+    });
+
     return allDueDatesReturn;
   }