NA values are repeatedly added in linked legend when switching layer off/on

[tim-salabim]

First of all many thanks for the grouped layer/legend feature. This really is a game changer as it makes interacting with the map much cleaner. I found a little bug related to NA value rendering in the legend.

If we use the new group argument in addLegend and the data has NAs I see that the na.color part of the legend gets repeatedly added to the legend as the layer is switched on/off.

library(leaflet)
library(mapview)
library(sf)

breweries = st_join(breweries, franconia[, "district"])

pal = colorFactor(c("red", "purple", "blue"), 
                  domain = mapview::breweries$district, 
                  na.color = "#BEBEBE")

leaflet() %>%
  addTiles() %>%
  addCircleMarkers(data = breweries, color = ~pal(district), group = "breweries")  %>%
  addLayersControl(overlayGroups = "breweries") %>%
  addLegend(pal = pal, values = breweries$district, group = "breweries")

In the primary rendering of the legend everything looks as expected. If we switch the breweries layer off and on we get a second NA entry in the legend. This is repeatedly added every time we switch the layer off/on.