"Subscript out of bounds": Shiny renderPlot() with for loop to generate plots

Question

I'm trying to use a for loop to create scatter plots. list_grps represents a number of groups which contain data (i.e. each group in the list contains some number of columns from a dataframe). I'm then applying a function to create scatterplots over these lists and saving the output to a list (list_scatter_plots). When I try to renderPlot() for my plots, I get the message "subscript out of bounds". The trace from shiny has the following additional details:

SEND {"errors":{"scatter":{"message":"subscript out of bounds","call":"grobs[[i]]","type":null}},"values":{},"inputMessages":[]}

Sample dataset: https://docs.google.com/spreadsheets/d/1j63zpOKBFFtuNTWga3Da7ytUnBfwGRVucGIFUENVsQQ/edit?usp=sharing

This is the code:

library(shiny) library(rio) library(DT) library(tidyverse) library(ggpubr)

ui <- navbarPage("App Name",
                 tabPanel("File Upload",
                          sidebarLayout(
                            sidebarPanel(
                              tags$h1(tags$b("File Upload")),
                              fileInput("file1",
                                        "Choose File",
                                        multiple = FALSE,
                                        accept = c("text/csv",
                                                   "text/tab-separated-values",
                                                   ".csv",
                                                   "text/tsv",
                                                   "text/comma-separated-values",
                                                   ".tsv")),
                              placeholder = "No file selected",
                              tags$hr(),
                              selectInput("dftype", label = "Data Type",
                                          choices = c(Phosphoproteome = "pProt",
                                                      Proteome = "Prot"),
                                          selected = "Prot"),
                              tags$hr(),
                              tags$h1(tags$b("Select Groups")),
                              tags$p("Select columns on table, then press 'Create new biological group' to group selected samples."),
                              actionButton("cr_grp", "Create new biological group")
                            ),
                            mainPanel(
                              DT::dataTableOutput(outputId = "exp_df")
                            )
                          )),
tabPanel("Quality Control"
                         sidebarPanel(

                         ),
                         mainPanel(
                           plotOutput(outputId = "scatter")
                         )
                ))

server <- function(input, output, session) {
  exp_dff <- eventReactive(input$file1,{
    infile <- input$file1
    req(infile)
    return(select(import(infile$datapath), -starts_with(c("Boost", "Empty", "MaxPepProb", "ReferenceIntensity"))))
  })
output$exp_df <- DT::renderDataTable(exp_dff())
  
  list_grps <- list()

  observeEvent(input$cr_grp, {
    append(list_grps, input$exp_df_columns_selected)
  })

list_scatter_plots <- lapply(list_grps, function(x) pairs(x, pch=20, col=“red”))

 output$scatter <- renderPlot(ggarrange(plotlist = list_scatter_plots))
}

shinyApp(ui=ui, server=server)