Editor: Preview pure skeleton. (#22858)

* EditorPreviewPureSkeleton

* Prevent errors caused by no parent.

* Simplify codes.

editor/js/Editor.js

@@ -424,6 +424,10 @@ Editor.prototype = {
 
 					helper = new THREE.SkeletonHelper( object.skeleton.bones[ 0 ] );
 
+				} else if ( object.isBone && ( object.parent?.isBone !== true ) ) {
+
+					helper = new THREE.SkeletonHelper( object );
+
 				} else {
 
 					// no helper for this object type