Internal: Used the new EditorUI#setEditableElement() API in the Class…

…icEditorUI class.

src/classiceditorui.js

@@ -84,7 +84,7 @@ export default class ClassicEditorUI extends EditorUI {
 
 		// Register the editable UI view in the editor. A single editor instance can aggregate multiple
 		// editable areas (roots) but the classic editor has only one.
-		this._editableElements.set( editable.name, editableElement );
+		this.setEditableElement( editable.name, editableElement );
 
 		// Let the global focus tracker know that the editable UI element is focusable and
 		// belongs to the editor. From now on, the focus tracker will sustain the editor focus