R Shiny problem with inputs belonging to a bsCollapsePanel - shiny

Question

In this shiny App (code below), tabPanel 'Scatter plot', note that the plot is correctly rendered only when the user expand the bsCollapsePanel 'Marker settings' for the first time. Before expanding the panel 'Marker settings' at first time, the message Error: argument is of length zero is shown. Can someone find out where the error is in the code?
library(shiny)
library(shinyBS)
library(tidyverse)
shinyApp(
ui = fluidPage(
tabsetPanel(
tabPanel("mtcars",
dataTableOutput("mtcarsDATA")),
tabPanel("Scatter plot",
sidebarPanel(
bsCollapse(id = "Side panel", open = "Variables",
bsCollapsePanel("Variables",
uiOutput("varx"),
uiOutput("vary"))
)
),
mainPanel(
bsCollapsePanel("Marker settings",
uiOutput("showMrk"),
uiOutput("shpMrk"),
uiOutput("forPorForma"),
uiOutput("forPorVar"),
uiOutput("mrkTrsp")),
plotOutput('SctPlot'))
)
)
),
server <- function(input, output) {
output$mtcarsDATA <- renderDataTable({
data <- mtcars
getModel <- reactive({
names(data) })
output$varx <- renderUI({
selectInput("varsel.x", HTML("Select var X<span style='color: red'>*</span>"),
choices = as.list(getModel()), multiple = F) })
getModelnum <- reactive({
filterNumeric <- data[sapply(data, is.numeric)]
names(filterNumeric) })
output$vary <- renderUI({
selectInput("varsel.y", HTML("Select var Y<span style='color: red'>*</span>(numerical only)"),
choices = as.list(getModelnum()), multiple = F) })
output$showMrk <- renderUI({
checkboxInput("show_Mrk", "Show marker", value=T) })
output$shpMrk <- renderUI({
conditionalPanel(condition = "input.show_Mrk == T",
radioButtons("shp_Mrk", "Format marker",
choices = c("by shape", "by variable"))) })
output$forPorForma <- renderUI({
conditionalPanel(condition = "input.shp_Mrk == 'by shape' & input.show_Mrk == T",
sliderInput("for_PorForma", 'Deslize para mudar o formato do marcador',
min = 1, max=25, value = 16)) })
output$mrkTrsp <- renderUI({
conditionalPanel(condition = "input.show_Mrk == T",
sliderInput("mrk_Trsp", 'Slide to change marker transparency',
min = 0, max=1, value = .5, step=.05)) })
getModelcat <- reactive({
filterCaracter <- data[sapply(data, is.character)]
names(filterCaracter) })
output$forPorVar <- renderUI({
conditionalPanel(condition = "input.show_Mrk == 1 & input.shp_Mrk == 'by variable'",
selectInput("forPorVar.sel", "Select var",
choices = as.list(getModelcat()), multiple = F)) })
output$SctPlot <- renderPlot({
if(input$show_Mrk == T){
if(input$shp_Mrk == "by shape") {
geomPoint <- geom_point(alpha=1-input$mrk_Trsp, shape=input$for_PorForma) } else {
geomPoint <- geom_point(alpha=1-input$mrk_Trsp, aes_string(shape=(input$forPorVar.sel))) }} else {
geomPoint <- geom_point(alpha=0) }
p <- data %>%
ggplot(aes_string(x=input$varsel.x, y=input$varsel.y)) +
geomPoint
p
})
data
})
}
)