How do I pass NULL as a Variable Value in RSHINY?

In phyloseq, there is a plot called plot_net.
The most basic plot_net plot code looks like this:

data(enterotype)
#Eliminate samples with no entereotype denomination
enterotype = subset_samples(enterotype, !is.na(Enterotype))

plot_net(enterotype, maxdist = 0.1, point_label = NULL)

I am trying to create an RShiny app which allows a user to alter this graphic.

point_label has several different options (ex: "SecTech", "SampleID", NULL).

I already have all of the other values for this label, I am just not sure how to add NULL.

Here is what I did:

This might not run since it isn't in a shiny app but I included it as an example to illustrate the issue.

library(shiny)
library(phyloseq)

# Data: This data contains info about nodes and edges on Phyloseq data.
data(enterotype)
#Eliminate samples with no entereotype denomination. Make it a lesson to 
always catalogue data correctly from the start. 
enterotype = subset_samples(enterotype, !is.na(Enterotype))

# a is the collection of variable names for point_label

a <- sample_variables(enterotype)

theme_set(theme_bw())

# Define UI for application that draws a network plot
shinyUI(fluidPage(

 # Application title
 titlePanel("Network Plots"),


 sidebarLayout(
 sidebarPanel(


 selectInput("labelBy",
 "Select the point label category",
 ***choices = c(a, "NA" = NULL),***
 selected = "NA")
 ),

 # Show a plot of the generated distribution
 mainPanel(
 plotOutput("netPlot")#,
 #plotOutput("networkPlot")
 )
 )
))

shinyServer(function(input, output) 

 output$netPlot <- renderPlot(

 plot_net(enterotype, maxdist = .1, point_label = input$labelBy)

 )
)
shinyApp(ui = ui, server = server)

This line is my question:

choices = c(a, "NA" = NULL)

How do I add NULL to my list of choices. No matter how I tried it, NULL was always taken as a zero value and it does not appear as an option.

If I write NULL as "NULL', the phyloseq function plot_net doesn't take it.
It only takes the value point_label = NULL for no value.

I think that it is possible to create an if... else loop where if a user clicks NULL on a checkboxInput then the plot will be generated by a second line of code specifying that the value in point_label is NULL, but that can be really cumbersome if there are several variables with a possible NULL Value.

There probably is some obvious trick like placing a $ or % in front of the NULL value but I couldn't find it. If anyone could help it would be great!

I don't think there is a way to use NULL in selectInput. Here's an alternative which you almost worked out - Use "None" (or any other replacement value) in selectInput and switch it with NULL while plotting. This way you don't have to write multiple if...else.

# update on UI side
selectInput("labelBy",
 "Select the point label category",
 choices = c("None", a),
 selected = "None")

# update on server side
output$netPlot <- renderPlot(
 point_labels <- switch(input$labelBy, "None" = NULL, input$labelBy)
 plot_net(enterotype, maxdist = .1, point_label = point_labels)
)