In my web I do not use Header Background Image in the customizer. If there is no image in console log appears an undefined error. If I put an image the error disappears.
I have been analyzing it and the undefined is generated in nprogress.js.
I hope for the next update instead of hiding the bar with a display: none there is any option in the customizer to enable or disable this feature in a real way. Not only hiding it as it actually continues to run.
Running code when it is not necessary seems silly to me.