WebGLRenderer: Check geometry type and avoid breakage when undefined.

src/renderers/WebGLRenderer.js

@@ -4689,7 +4689,7 @@ THREE.WebGLRenderer = function ( parameters ) {
 
 					addBuffer( scene.__webglObjects, geometry, object );
 
-				} else {
+				} else if ( geometry instanceof THREE.Geometry ) {
 
 					for ( g in geometry.geometryGroups ) {