Hi Guys, I had the same problem. I had my coded stuff and plugins on my sidebars and when I wanted to add just one widget, everything disappeared and only widget remained.
Here is how I solved it:
I created dynamic sidebar as described here
https://www.balkhis.com/web-designs-resources/how-to-add-a-dynamic-sidebar-in-wordpress/
and then added it to my existing sidebar at the place that I wanted my widget.
If you have further questions I will be glad to answer.
