JSONLoader: Added check for metadata object. Fixes #4511.

src/loaders/JSONLoader.js

@@ -42,7 +42,7 @@ THREE.JSONLoader.prototype.loadAjaxJSON = function ( context, url, callback, tex
 
 					var json = JSON.parse( xhr.responseText );
 
-					if ( json.metadata.type === 'scene' ) {
+					if ( json.metadata !== undefined && json.metadata.type === 'scene' ) {
 
 						console.error( 'THREE.JSONLoader: "' + url + '" seems to be a Scene. Use THREE.SceneLoader instead.' );
 						return;