modelq | code generator for creating Golang codes | Database library

Without having some data at hand to reproduce your app its hard to spot the error and makes it more of a guessing game.

So my first guess would be, that you're not handling the reactivity in your app correctly. You're render* functions are calling reactive values, but when no file was uploaded and no Regressor was chosen, there is no data to work with, so it will throw an error. I would suggest checking out the Shiny principles of reactivity and the functions observeEvent, req, validate.

Then you can remove the lines with is.null(*), as the req() function does that for you.

Next thought, what is mpg? And I think you should change summary(modelq()) to summary(modelq) too, as you just want to print the summary of the object and not call the object.

Calling library(shiny) once is enough. ;)

In the prediction you are also calling df, which is not assigned in that function. I added a reactiveValues object, where you can save other objects in a reactive way. And in your second renderUI we add the data to the reactiveValues object like thisdf$file <- df. And in the prediticion function you can call the data like this predict(modelq,df$file).

I also changed the regformula to an eventReactive() which will be executed every time you press the action button.

And lastly, I changed to formula creation to as.formula(paste("mpg",'~',paste(input$independents, collapse= "+"))), as if you only call input$independents, it will only take the first argument.