[FIX] web_calendar: Instantiate moment correctly

Without instantiating the moment it will throw errors to some users.
Desired behavior after PR is merged: No longer undefined errors for end users
Fixes #18994
Fixes #18618

addons/web_calendar/static/src/js/web_calendar.js

@@ -54,7 +54,7 @@ function get_fc_defaultOptions() {
         monthNamesShort: moment.monthsShort(),
         dayNames: moment.weekdays(),
         dayNamesShort: moment.weekdaysShort(),
-        firstDay: moment._locale._week.dow,
+        firstDay: moment()._locale._week.dow,
         weekNumberCalculation: function(date) {
             return moment(date).week();
         },