views/week-header: Check for NULL child

I'm not sure if this is actually correct, but I'm too tired to
think the week header through, so let's just be slightly more
protective for now.

src/gui/views/gcal-week-header.c

@@ -764,6 +764,9 @@ apply_overflow_at_weekday (GcalWeekHeader *self,
 
   child = gtk_grid_get_child_at (self->grid, weekday, 3);
 
+  if (!child)
+    return;
+
   split_event_widget_at_column (self, child, weekday);
   gtk_widget_set_visible (child, FALSE);
 }