fix #3942

Closes #3942

Merge request studip/studip!2801

lib/models/Courseware/BlockTypes/Text.php

@@ -192,7 +192,7 @@ class Text extends BlockType
 
             }
 
-            return array();
+            return null;
         });
     }
 }

lib/models/Courseware/ContainerTypes/ContainerType.php

@@ -199,7 +199,7 @@ abstract class ContainerType
             foreach ($section['blocks'] as &$block) {
                 $block = $block_map[$block] ?? null;
             }
-            $section['blocks'] = array_filter($section['blocks']);
+            $section['blocks'] = array_values(array_filter($section['blocks']));
         }
 
         return $payload;