library(leafletR)
library(leaflet)


m <- leaflet() %>% setView(lng = -0.1, lat = 51.5, zoom = 10)
m %>% addProviderTiles(providers$Esri.NatGeoWorldMap)
m %>% addProviderTiles(providers$Stamen.Toner)
m %>% addProviderTiles(providers$Stamen.TonerLines,
        options = providerTileOptions(opacity = 0.35)) %>%
  addProviderTiles(providers$Stamen.TonerLabels)



> m <- leaflet() %>% setView(lng = -0.1, lat = 51.5, zoom = 10)
> m %>% addProviderTiles(providers$Esri.NatGeoWorldMap)
Error in browseURL(x, ...) : 'url' must be a non-empty character string
> m %>% addProviderTiles(providers$Stamen.Toner)
Error in browseURL(x, ...) : 'url' must be a non-empty character string
> m %>% addProviderTiles(providers$Stamen.TonerLines,
+         options = providerTileOptions(opacity = 0.35)) %>%
+   addProviderTiles(providers$Stamen.TonerLabels)
Error in browseURL(x, ...) : 'url' must be a non-empty character string
>