Resolve "Allow using time slot numbers in long term absences dialog (instead of datetimes)"

Closes #300 (closed)

aleksis/apps/alsijil/frontend/components/coursebook/absences/AbsenceCreationForm.vue

@@ -186,16 +186,19 @@ export default {
     getPeriodsForWeekday(weekday) {
       // Adapt from python conventions
       const pythonWeekday = weekday - 1;
-      return this.periodsByDay.find(
+      let periodsForWeekday = this.periodsByDay.find(
         (period) => period.weekday === pythonWeekday,
-      ).periods;
+      );
+      if (!periodsForWeekday) return false;
+      return periodsForWeekday.periods;
     },
     handleStartDate(date) {
       this.start = DateTime.fromISO(date);
 
-      if (this.periodsByDay.length > 0) {
+      if (this.periodsByDay && this.periodsByDay.length > 0) {
         // Select periods for day
         this.startPeriods = this.getPeriodsForWeekday(this.start.weekday);
+        if (!this.startPeriods) return;
         // Sync PeriodSelect
         const startTime = this.start.toFormat("HH:mm:ss");
         this.startSlot = this.startPeriods.find(
@@ -206,9 +209,10 @@ export default {
     handleEndDate(date) {
       this.end = DateTime.fromISO(date);
 
-      if (this.periodsByDay.length > 0) {
+      if (this.periodsByDay && this.periodsByDay.length > 0) {
         // Select periods for day
         this.endPeriods = this.getPeriodsForWeekday(this.end.weekday);
+        if (!this.endPeriods) return;
         // Sync PeriodSelect
         const endTime = this.end.toFormat("HH:mm:ss");
         this.endSlot = this.endPeriods.find(