[player] Fix restoring of drop lists

Change creation of the list of possible drop list values.
The values can be found at different hierarchy levels in the unit
definition, as they can be sub elements of cloze elements

projects/player/src/app/services/unit-state-element-mapper.service.ts

@@ -3,7 +3,6 @@ import { Unit } from '../../../../common/models/unit';
 import {
   DragNDropValueObject, InputElement, InputElementValue, UIElement
 } from '../../../../common/models/uI-element';
-import { Section } from '../../../../common/models/section';
 import { ImageElement } from '../../../../common/ui-elements/image/image-element';
 import { VideoElement } from '../../../../common/ui-elements/video/video-element';
 import { AudioElement } from '../../../../common/ui-elements/audio/audio-element';
@@ -18,16 +17,32 @@ export class UnitStateElementMapperService {
   dropListValueIds!: DragNDropValueObject[];
 
   registerDropListValueIds(unitDefinition: Unit): void {
-    this.dropListValueIds = unitDefinition.pages.reduce(
-      (accumulator: Section[], currentValue) => accumulator.concat(currentValue.sections), []
-    ).reduce(
-      (accumulator: UIElement[], currentValue) => accumulator.concat(currentValue.elements), []
-    ).filter(element => element.type === 'drop-list').reduce(
-      (accumulator: DragNDropValueObject[], currentValue: UIElement) => (
-        (currentValue.value && currentValue.value.length) ? accumulator.concat(currentValue.value) : accumulator), []
-    );
+    const dropListElements: UIElement[] = [];
+    this.findNestedDropLists(unitDefinition.pages, dropListElements);
+    this.dropListValueIds = dropListElements
+      .reduce(
+        (accumulator: DragNDropValueObject[], currentValue: UIElement) => (
+          (currentValue.value && currentValue.value.length) ? accumulator.concat(currentValue.value) : accumulator), []
+      );
   }
 
+  private findNestedDropLists(value: any | unknown[], dropListElements: UIElement[]): void {
+    if (this.isObject(value)) {
+      if (value.type === 'drop-list') {
+        dropListElements.push(value);
+      } else {
+        const keys = (Object.keys(value));
+        keys.forEach((key: string) => this.findNestedDropLists(value[key], dropListElements));
+      }
+    } else if (this.isArray(value)) {
+      value.forEach((element: unknown) => this.findNestedDropLists(element, dropListElements));
+    }
+  }
+
+  private isObject = (value: unknown) => !!(value && typeof value === 'object' && !Array.isArray(value));
+
+  private isArray = (value: unknown) => !!(value && typeof value === 'object' && Array.isArray(value));
+
   mapToElementValue(
     elementModel: UIElement,
     unitStateElement: UnitStateElementCode | undefined,