Fixes problem with latest versions of ICU library where AmPmMarkers is no longer defined for each locale. If that's the case fall back to AmPmMarkersAbbr and if that one doesn't exist, fallback to null. This fixes a second problem where previously we always returned 'null' due to the index being a string instead of an integer. That's also fixed now.

Author: hostep

lib/internal/Magento/Framework/View/Element/Html/Calendar.php

@@ -111,8 +111,15 @@ protected function _toHtml()
         $this->assignFieldsValues($localeData);
 
         // get "am" & "pm" words
-        $this->assign('am', $this->encoder->encode($localeData['calendar']['gregorian']['AmPmMarkers']['0']));
-        $this->assign('pm', $this->encoder->encode($localeData['calendar']['gregorian']['AmPmMarkers']['1']));
+        // AmPmMarkers and AmPmMarkersAbbr aren't guaranteed to exist, so fallback to null if neither exist
+        $amWord = $localeData['calendar']['gregorian']['AmPmMarkers'][0] ??
+                  $localeData['calendar']['gregorian']['AmPmMarkersAbbr'][0] ??
+                  null;
+        $pmWord = $localeData['calendar']['gregorian']['AmPmMarkers'][1] ??
+                  $localeData['calendar']['gregorian']['AmPmMarkersAbbr'][1] ??
+                  null;
+        $this->assign('am', $this->encoder->encode($amWord));
+        $this->assign('pm', $this->encoder->encode($pmWord));
 
         // get first day of week and weekend days
         $this->assign(

lib/internal/Magento/Framework/View/Test/Unit/Element/Html/CalendarTest.php

@@ -48,6 +48,9 @@ public function localesDataProvider()
             ['en_US'],
             ['ja_JP'],
             ['ko_KR'],
+            ['lv_LV'],
+            ['sv_SE'],
+            ['de_AT'],
         ];
     }