ThumbPosition cannot accept the arguments

Hi everyone,
I am trying to read some JSON data. That works fine, no issues at all, until it runs into an error. Even though I check for a lot of errors it shows this error:

ThumbPosition cannot accept the arguments

followed by:

, not found

now I know what the issue is but I am wondering what ThumbPosition is. Does anyone know about it?

Do you feed a slider from json’s values ? For example