Consider the case when vrdisplaypresentchange fires but device is null.

src/renderers/webvr/WebVRManager.js

@@ -43,7 +43,7 @@ function WebVRManager( renderer ) {
 
 	function onVRDisplayPresentChange() {
 
-		if ( device.isPresenting ) {
+		if ( device !== null && device.isPresenting ) {
 
 			var eyeParameters = device.getEyeParameters( 'left' );
 			var renderWidth = eyeParameters.renderWidth;