How to Create a Folder in Project using Custom IWizard in VSIX

Question

i am creating a custom wizard, i want to create a folder inside my project and add some files into this folder. i used this codes

Project project = _dte.Solution.Projects.Item(1);
project.ProjectItems.AddFolder("Common");

but i get a null refrence exception.

can someone tell me how i can create a folder for my project?