Better error message needed when points and grid do not overlap

[mpjashby]

Describe the bug
When the grid does not overlap the points, a better error message is needed. Currently the message is Error in sum(counts$n): invalid 'type' (list) of argument.

To Reproduce

library(sf)
library(sfhotspot)
library(tidyverse)


# Loading Data ----------------------------------------------------

# ASB incidents in Lancashire in 2020 
asb <- read_tsv("https://mpjashby.github.io/crimemappingdata/lancashire_asb.tab") |>
	st_as_sf(coords = c("longitude","latitude"), crs = "EPSG:4326") |>
	st_transform("EPSG:27700")

# Lancashire districts
districts <- read_sf("https://mpjashby.github.io/crimemappingdata/northumbria_districts.geojson") |>
	st_transform("EPSG:27700")

# Calculate KDE values for Lancashire
asb_kde_Lancashire <- asb |>
	hotspot_kde(grid = hotspot_grid(districts, cell_size = 5000))

Expected behavior
An error message that explains the problem and how to fix it.