Shiny dependent tabs

I’m using shiny dashboard to create a GUI. My first tab in the sidebar will contain browsing function which uploads an excel file.
I’d like the second tab to contain a list using select button. The list is created by extracting data from the uploaded file. How do I put on hold the “select” button, until the first step is done?

Best

When loading the app it gives me an error since the excel file wasn't uploaded yet in the first tab.
Does anyone have any experience with this?

shiny has functions req()
as well as validate() and need() for handling requirements on downstream reactives.
I think in your case you would just add an appropraite req() call.
look at ?req in console for the documentation

Thank you for replying I'll try to see if that helps.

Best