First of all, thank you very much for this plugin. The idea is simply splendid.
I've installed your plugin in three blogs (different servers, different settings and different plugins):
Your plugin only works as expected in the third one. In the first and the second, a blank file is produced.
I've activated WordPress debug mode in the first blog; after executing the plugin, I get many error warnings. I can't transcribe them here because of security reasons, but I could send them to you if you give me and email address.
On the other hand, when the blog is located in a subdirectory, once created the widget_data.json file, the backend is redirected to a non existing page.