R Shiny - Select multiple xlsx files, rbind them, filter dataset before generating a plot from the multiple xlsx files selected

Am trying to generate an R Shiny app where the user can select multiple xlsx files, stack them on top of each other using rbind function, filter the dataset before generating a plot.

The code in which I am getting an error is as follows:

ui <- fluidPage(
    titlePanel("Charts"),
    sidebarLayout(
        sidebarPanel(
            fileInput(inputId="files", label = "Select XLSX files", multiple = TRUE))),
        mainPanel(
            tabsetPanel(
                tabPanel("Plot", plotOutput("plot1"))
    ))))

server <- function(input,output){
    data <- reactive({
        data=lapply(input$files$datapath, read_xlsx)
        data <- do.call(rbind, data)

        if(is.null(data)){
            return(NULL)
        }

        data %>% 
            filter(Variable %in%   c('A','B,'C')) %>% 
            select(Variable, `Name`,`Calconc`) %>% 
            filter(Name %in% c('D','E','F','G')) %>% 
    })
    output$plot1 <- renderPlot({
        data() %>% 
            filter(Variable=="A") %>% 
            ggplot(aes(x = Name, y = Calconc))         
    })
}
shinyApp(ui=ui, server=server)

Error:
Warning: Error in UseMethod: no applicable method for 'filter' applied to an object of class "NULL"

Here is how we could do it:

I have created two dummy xlsx files:

df1 <- data.frame(
  Variable = c("A", "B", "C", "D", "E", "F", "G"),
  Name = c("Name1", "Name2", "Name3", "Name4", "Name5", "Name6", "Name7"),
  Calconc = c(0.5, 1.2, 0.9, 1.8, 2.3, 3.1, 4.5)
)

df2 <- data.frame(
  Variable = rev(c("A", "B", "C", "D", "E", "F", "G")),
  Name = rev(c("Name1", "Name3", "Name1", "Name4", "Name1", "Name6", "Name7")),
  Calconc = c(8.5, 9.2, 0.8, 1.8, 2.3, 3.1, 8.5)
)

library(writexl)

write_xlsx(df1, "df1.xlsx")
write_xlsx(df2, "df2.xlsx")

I also tweaked a little your code: Now we have a tab were the xlsx data appear.
To get a plot I commented out the filter funcntions:

library(shiny)
library(readxl)
library(dplyr)
library(ggplot2)

ui <- fluidPage(
  titlePanel("Charts"),
  sidebarLayout(
    sidebarPanel(
      fileInput(inputId="files", label = "Select XLSX files", multiple = TRUE)
    ),
    mainPanel(
      tabsetPanel(
        tabPanel("Plot", plotOutput("plot1")
                 ),
        tabPanel("data", DT::DTOutput("data"))
      )
    )
  )
)

server <- function(input, output) {
  data <- reactive({
    req(input$files)
    data <- lapply(input$files$datapath, read_xlsx)
    if(length(data) == 0) {
      return(data.frame()) # return empty data frame when no files are selected
    }
    data <- do.call(rbind, data)
    data %>%
     # filter(Variable %in% c('A','B','C')) %>%
      select(Variable, Name, Calconc) # %>%
     # filter(Name %in% c('D','E','F','G'))
  })
  
  output$plot1 <- renderPlot({
    data() %>%
      filter(Variable == "A") %>%
      ggplot(aes(x = Name, y = Calconc)) +
      geom_point()
  })
  
  output$data <- DT::renderDT({
    data()
  })
}

shinyApp(ui=ui, server=server)