Don't try to close webpack watcher if it doesn't exist.

lib/middleware/webpack.js

@@ -38,7 +38,9 @@ module.exports = function(options) {
   })
 
   lifecycle.observe(function() {
-    watching.watcher.close()
+    if (watching.watcher) {
+      watching.watcher.close()
+    }
   })
 
   function doneListener(stats) {