message: Handle case of treeForApp returning undefined
  As it does when used in
diff: @@ -62,8 +62,11 @@ module.exports = {
  treeForApp() {
  let tree = this._super.treeForApp.apply(this, arguments);
  -
  - return mergeTrees([this.automaticReexports(), tree], {
  + let trees = [this.automaticReexports()];
  + if (tree) {
  + trees.push(tree);
  + }
  + return mergeTrees(trees, {
  overwrite: true,
  });
  },
  