Fix "Cannot call method add on possibly null value" bug pointed by Psalm

src/Utilities/Date.php

@@ -120,8 +120,11 @@ class Date
 
                 if (null !== $lastMonth) {
                     $dateStart = $lastMonth->getEndDate();
+
+                    if (null !== $dateStart) {
                         $dateStart->add(new DateInterval('P1D'));
                     }
+                }
 
                 if (null !== $nextMonth) {
                     $dateEnd = $nextMonth->getStartDate();