Using function 'ModelPlot' from 'mixexp' package in R shiny trying to draw a reactive plot but failed

I have 6 variables for a mixture design of experiment and fitted a lm model with interactions. Trying to use 'ModelPlot' function for contour plot in shiny and let user choose which 3 variables to be in plot and the other 3 to hold fixed values in slice argument, however it does not work and giving me errors. Can someone help with it? Or is this function not shiny-friendly, what would be a better way to do this?

library(shiny)
library(dplyr)
library(mixexp)

load(mix_model)
var_choices = c("f1", "f2", "f3", "f4", "f5", "f6")

Define UI for application that draws a contour plot

ui <- fluidPage(

titlePanel("Design of Experiment Plot"),

sidebarLayout(
sidebarPanel(

  h3("Select variables for contour plot"),
  
  fluidRow(
    column(6,
           selectInput("in_f1",
                       "assign a variable as f1:",
                       choices = var_choices,
                       selected = "f1")),
    column(6,
           sliderInput("range_f1", "range for f1:",
                       min = 0, max = 0.5, value = c(0, 5),
                       dragRange = TRUE, sep = ""))),
  
  fluidRow(
    column(6,
           selectInput("in_f2",
                       "assign a variable as f2:",
                       choices = var_choices,
                       selected = "f2")),
    column(6,
           sliderInput("range_f2", "range for f2:",
                       min = 0, max = 0.5, value = c(0, 5),
                       dragRange = TRUE, sep = ""))),
  
  fluidRow(
    column(6,
           selectInput("in_f3",
                       "assign a variable as f3:",
                       choices = var_choices,
                       selected = "f3")),
    column(6,
           sliderInput("range_f3", "range for f3:",
                       min = 0, max = 0.5, value = c(0, 5),
                       dragRange = TRUE, sep = ""))),
  
  fluidRow(
    column(6,
           selectInput("in_f4",
                       "assign a variable as f4:",
                       choices = var_choices,
                       selected = "f4")),
    column(6,
           sliderInput("val_f4", 
                       "fix value for f4:",
                       min = 0, max = 0.5, value = 0, step = 0.01))),
  
  fluidRow(
    column(6,
           selectInput("in_f5",
                       "assign a variable as f5:",
                       choices = var_choices,
                       selected = "f5")),
    column(6,
           sliderInput("val_f5", 
                       "fix value for f5:",
                       min = 0, max = 0.5, value = 0,  step = 0.01))),
  
  fluidRow(
    column(6,
           selectInput("in_f6",
                       "assign a variable as f6:",
                       choices = var_choices,
                       selected = "f6")),
    column(6, 
           sliderInput("val_f6", 
                       "fix value for f6:",
                       min = 0, max = 0.5, value = 0, step = 0.01)))
  
  
),



mainPanel(
  plotOutput("contourplot")
)

)
)

Define server logic

server <- function(input, output) {

output$contourplot <- renderPlot ({

mix_plot <- ModelPlot (
  model = mix_model,
  dimensions = list(x1 = input$in_f1, x2 = input$in_f2, x3 = input$in_f3),
  slice = list(mix.vars = c(input$in_f4 = input$val_f4, input$in_f5 = input$val_f5, input$in_f6 = input$val_f6)),
  lims=c(input$range_f1,input$range_f2,input$range_f3),
  main = paste("Contour Plot"),
  constraints = TRUE, contour = TRUE, fill = TRUE,
  axislabs = c(input$in_f1, input$in_f2, input$in_f3),
  cornerlabs = c("X1", "X2", "X3"),
  pseudo = FALSE
)

})

}

shinyApp(ui = ui, server = server)