Heroku[router] making GET request to "/assets/bundle.js.map" - ruby-on-rails

Question

Getting a weird error in my heroku logs:
heroku[router]: at=info method=GET path="/assets/bundle.js.map" host=syndicator1.herokuapp.com request_id=106a2ab1-c373-4e2c-817a-f740a2e0819b fwd="24.102.93.192" dyno=web.1 connect=1ms service=23ms status=404 bytes=1902 protocol=https
app[web.1]: Started GET "/assets/bundle.js.map"
ActionController::RoutingError (No route matches [GET] "/assets/bundle.js.map"):
I have no idea where this is coming from, I just know that my application isn't behaving the way it is in development. Any idea where to start fixing this?
EDIT:
The application works, mostly, in production. What doesn't work is the error-handling.
Here is what happens when I try to submit my form in development with invalid values:
Basically my Error-Box component gets triggered.
This is what happens when I do the same in production.
The Error-Box component does not get triggered. The 422 is correct, I want that but it should be triggering the error-box, instead I get the following in console:
Here is the code in the component that should trigger the error box,
handleSubmit(e) {
e.preventDefault();
const {addEvents,toggleModal,handleError} = this.props;
axios.post('/events', {
event: this.state
})
.then(function (response) {
const event = response.data.event;
toggleModal();
addEvents(event);
})
.catch(function (error) {
handleError(error.response.data);
});
}