Shiny - How change ActionButton to NULL?

I've some ActionButton in my DT:

shinyInput(actionButton, nrow(RV2()), 'button_', label = "OK",
                          onclick = 'Shiny.onInputChange(\"select_button\", this.id)')

When I click on the button I get:

  observeEvent(input$select_button, {
       print(input$select_button)
  })

If I don't click on the button input$select_button = NULL but after I click once it input$select_button = button_1 How I can reset it again to NULL?