Can't seem to use optional foreign key relationships when creating new object using Django+AngularJS

I'm using AngularJS on the front-end with Django managing the backend and API along with the Django REST framework package. I have a Project model which belongs to User and has many (optional) Intervals and Statements. I need to be able to create a 'blank' project and add any intervals/statements later, but I'm hitting a validation error when creating the project. Below are the relevant code sections.I'm using AngularJS on the front-end with Djang