Render results plot **before** destroying loading screen

[Felixmil]

Right now, loading screen is destroyed, then the result_screen is created (in which the plot is generated).

#main.R
    observeEvent(app_data$simulation_results, {
      message("Simulation result received. Destroying intro screen.")
      app_data$destroy_intro_screen <- TRUE
      result_screen$server("result_screen", app_data)
    })

It would be better to destroy the loading screen only after the plot is rendered.