Shiny Error: Error in match.arg(position) : 'arg' must be NULL or a character vector

I keep getting this error: "Error in match.arg(position) : 'arg' must be NULL or a character vector" when trying to run my shiny app. I have no idea whether the problem is in my ui or server code. Does anyone know what the issue is?

ui logic

library(rtweet)
library(tidytext)
library(tidyverse)
library(stringr)
library(shiny)
library(DT)
library(markdown)
library(shinythemes)

source("R_rainclouds.R")


#create variables for ggplot
joined_names_tweets <- read_rds("joined_names_tweets.rds")
tweets <- read_rds("tweets.rds")


ui <- navbarPage("Project",
                 theme = shinytheme("united"),
    
     ###########
     ###DATA###
     ##########
 
           tabPanel("Graphics",
                    tabsetPanel(
                      tabPanel("Over Time",
                               sidebarPanel(
                                 selectInput("screen_name", "NCAA Twitter Accounts:",
                                             choices = joined_names_tweets$screen_name),
                                 mainPanel(plotOutput("raincloud")))),
                      tabPanel("Stuff"))),
                    
    #############
    ##EXPLORE###
    ############
    
           tabPanel("Explore",
                    
                    fluidPage(
                      titlePanel("Explore the data"),
                      
                      sidebarLayout(
                        sidebarPanel(
                          helpText("Pick an NCAA Twitter Account to view recent tweets"),
                          h3("Tweet Search"),
                          selectInput("screen_name", NULL,
                                      choices = tweets$screen_name,
                                      selected = "@NCAA")),
                        mainPanel(
                          DTOutput("word_table")),
    ##########
    ##ABOUT##
    #########
                    tabPanel("About",
                    fluidRow(
                        column(8,
                               includeMarkdown("about.Rmd"))))))))

server logic

server <- function(input, output, session) {
 
  ########
  ##DATA##
  ########
  
  output$raincloud <- renderPlot({
    
    data <- joined_names_tweets %>%
      filter(screen_name == input$screen_name) %>% 
      
      ggplot(aes(x=sex_id,y=created_at, fill = sex_id)) +
      geom_flat_violin(position = position_nudge(x = .2, y = 0),adjust = 4) +
      geom_point(position = position_jitter(width = .15), size = .25, alpha = .5) +
      ylab('Date')+
      xlab('Gender')+
      coord_flip()+
      theme_cowplot()+
      guides(fill = FALSE) +
      scale_fill_manual(values = c("snow1", "steelblue"))
  })
  

  ############
  ##EXPLORE##
  ###########
  
  output$word_table <- renderDT({
    
    datatable(tweets %>% filter(screen_name == input$screen_name) %>% select(-screen_name),
              class = 'display',
              rownames = FALSE,
              selection = 'single',
              colnames = c("Tweet Text", "Date", "Favorites", "Retweets"),
              options = list(dom = 'tip'))
  })
      
}

      
# Run the application 
shinyApp(ui = ui, server = server)