Help with Filtering Data Based on Input of Nested SelectInputs

Hello Everyone,
I would love to have some help with this issue I am running into while creating a Shiny Dashboard.
Currently I have a subject filtering system in the UI that works like this:

There is one main selectInput (filter A) with options = 1,2,3,4
if option 1 is selected, nothing should happen (this will be for an overview of the data means)
if option 2 is selected, another selectInput will appear (filter B) with its own set of choices
if option 3 is selected, another selectInput will appear (filter C) with its own set of choices
if option 4 is selected, another selectInput will appear (filter D) with its own set of choices

Currently I have this working well in the UI. However, in the server I need to filter (I am using dplyr) the data based on either Filter A (if option 1 is picked), or one of the options within Filter B,C, or D (depending on which one is selected) to create a ggplot.

I am new to R and firstly don't have a great idea of how to do this. Secondly, I also run into the problem that for instance if a choice from Filter B is selected, Filter C and D will have no inputs and I get an error saying that a value of 0 can't be put through the filter system.

I really hope someone can help out with this! Thanks so much!
I look forward to chatting.

Here is some more information:

ui <- dashboardPage(

selectInput( "Category", "Select Category", c("Overview", "Position", "Age", "Player")),

server <- function(session, input, output) {

    categoryfilterinput<- paste(input$Category)
    output$outputfilter <- renderUI({
                  "Position" = selectInput('positioninput', 'Select Position', choices = c("Forwards","Defencemen", "Goalies")),
                  "Age" = selectInput('ageinput', 'Select Age Range', choices = c("<20","20-23", "23-25", "25-30", ">30")),
                  "Player" = selectInput("Playernameinput", "Select Player", choices = Roster$Playername))


Also, it is important to note that there are quite a few "NA"s in my data set. I unfortunately can not replace these with zeros so I must deal with them. which in this particular case has turned out to be challenging