Updated JSM.

examples/jsm/exporters/GLTFExporter.js

@@ -1668,6 +1668,9 @@ GLTFExporter.prototype = {
 			var node = outputJSON.nodes[ nodeMap.get( object ) ];
 
 			var skeleton = object.skeleton;
+
+			if ( skeleton === undefined ) return null;
+
 			var rootJoint = object.skeleton.bones[ 0 ];
 
 			if ( rootJoint === undefined ) return null;