xgboostExplainer | An R package that makes xgboost models | Machine Learning library

I am currently trying to work with the new xgboostExplainer package.

I am following the githib page here https://github.com/AppliedDataSciencePartners/xgboostExplainer/blob/master/R/explainPredictions.R

on line 34, the xgboost model is ran:

xgb.model <- xgboost(param =param, data = xgb.train.data, nrounds=3)

However on line 43 I am running into some problems.

explainer = buildExplainer(xgb.model,xgb.train.data, type="binary", base_score = 0.5, n_first_tree = xgb.model$best_ntreelimit - 1)

I understand that n_first_tree is depreciated but I cannot seem to access the xgb.model$best_ntreelimit -1 part.

The sections I can access in xgboost are;