Adding tests to video player code

JavaScript
210

This is what I have: https://jsfiddle.net/m8yekr61/

  describe("addCoverHandler", function() {
    it("needs a coverSelector parameter", function() {
      const fnCall = () => manageCover.addCoverHandler();
      expect(fnCall).toThrowError(/Cannot read properties of null/);
    });

What is the next thing I am supposed to do?