404 error when embedding Shiny app in a package

[bersbersbers]

R/bug.R:

ui <- shiny::fluidPage(
    shiny::titlePanel("Hello Shiny!"),
    shiny::mainPanel(),
    theme = shinythemes::shinytheme("flatly"),
)

server <- function(input, output) {}

#' @export
bug <- function() {
    shiny::runApp(shinyApp(ui, server))
}

DESCRIPTION:

Package: bug
Title: bug
Version: 0.0.0.0001
Encoding: UTF-8
RoxygenNote: 7.1.1

NAMESPACE:

# Generated by roxygen2: do not edit by hand

export(bug)

Repro steps:

R -e "devtools::install()"
R -e "bug::bug()"

Result: