NPE on Java 15

[boris-petrov]

I updated our project to use Java 15 and I started getting a bunch of:

Error: CSS: “top”: Cannot invoke "org.w3c.css.values.CssValue.getType()" because "this.val1" is null.

This seems to mostly/only happen on FullCalendar's CSS on, for example, this code (on the calc line exacty):

.fc-v-event:not(.fc-event-selected) .fc-event-resizer-start {
  top: -4px;
  top: calc(var(--fc-event-resizer-thickness, 8px) / -2);
}

[ylafon]

This is related to improper parsing of variables in complex values (like functions). Indeed a bug

[ylafon]

fixed by #316