How to fix "Unexpected end of JSON"

I had my users page displaying users but then when I stopped working on the code for a while and came back to it I am getting an error which says "Error while processing route: users Unexpected end of JSON input SyntaxError: Unexpected end of JSON input" and also "Uncaught SyntaxError: Unexpected end of JSON input"

I don't know where to start on trying to fix this issue.

Here is the front-end code in the route file users.js:

model() {
    return this.get("store").findAll("user");
  },

Here is the api code in the users controller:

# GET /users
  def index
    render jsonapi: User.all
  end