`attempt to apply non-function` error.

[trivialfis]

Hi, I'm trying to check the xgboost project but ran into the following error, which stops the check:

Error in desc$get_field("BugReports", default = character()) : 
  attempt to apply non-function
Calls: revdep_check ... lapply -> FUN -> cat_package_info -> paste0 -> pkg_github
Execution halted

I don't know why desc is null yet, but patching the pkg_github function with:

pkg_github <- function(desc) {
  if (is.null(desc)) {
    return (NA_character_)
  }
...
}

can help workaround the issue.