demo.js

define(['core/yui'], function(Y) {
  
  // ...
  
  function foo() {
    Y.use('core_filepicker', function (Y) {
      // ...
      M.core_filepicker.show(Y, options);
      // ^^^ if executed quick enough, this fails with 
      // TypeError: Y.Node.createWithFilesSkin(...) is null
      // lib/javascript.php/-1/repository/filepicker.js Line 1304
  }