Fix Skeleton.pose() bug (#8976)

* Fix Skeleton.pose() bug

* Clean up THREE.Skeleton

src/objects/Skeleton.js

@@ -123,7 +123,7 @@ Object.assign( THREE.Skeleton.prototype, {
 
 			if ( bone ) {
 
-				if ( bone.parent ) {
+				if ( bone.parent instanceof THREE.Bone ) {
 
 					bone.matrix.getInverse( bone.parent.matrixWorld );
 					bone.matrix.multiply( bone.matrixWorld );