Callbacks receive initial Null or NaN values and proper use of `PreventUpdate()` calls

[stu]

I believe that if you provide an MRE, people here can help you more easily.

How to write a Minimal Reproducible Example (MRE)

Adding a minimal reproducible example to your questions will help attract community members to answer your question. The goal of writing a MRE is to allow other members of the community to run the code on their computers and “reproduce” the error, so that they can best help you. Please make sure that your MRE includes data and that the code is correctly pre-formatted. To preformat, copy and paste the code in your post and then click on </> (Preformatted text), as shown on the image: [pre-for…