Validates that vector has length greater than 0

validate_has_elements(x, msg)

data <- data.frame(
  id = c(1:10, 11:20, 1:10),
  strata = rep(c("A", "B"), each = 15)
)
ui <- fluidPage(
  selectInput("ref1", "Select strata1 to compare",
    choices = c("A", "B", "C"), selected = "A"
  ),
  selectInput("ref2", "Select strata2 to compare",
    choices = c("A", "B", "C"), selected = "B"
  ),
  verbatimTextOutput("arm_summary")
)

server <- function(input, output) {
  output$arm_summary <- renderText({
    sample_1 <- data$id[data$strata == input$ref1]
    sample_2 <- data$id[data$strata == input$ref2]

    validate_has_elements(sample_1, "No subjects in strata1.")
    validate_has_elements(sample_2, "No subjects in strata2.")

    paste0(
      "Number of samples in: strata1=", length(sample_1),
      " comparions strata2=", length(sample_2)
    )
  })
}
if (interactive()) {
  shinyApp(ui, server)
}