[Bug]: Tuning with default `tune_settings` fails

[MalteKurz]

Describe the bug

Tuning with default tune_settings fails (even after fixing #155). If tune_settings$measure is set to NULL according to the docu (https://docs.doubleml.org/r/stable/reference/DoubleML.html#method-tune) default measures should be used.

Minimum reproducible code snippet

library(DoubleML)
library(mlr3)
library(mlr3learners)
library(data.table)
set.seed(2)
ml_g = lrn("regr.ranger", num.trees = 10, max.depth = 2)
ml_m = ml_g$clone()
obj_dml_data = make_plr_CCDDHNR2018(alpha = 0.5)
dml_plr_obj = DoubleMLPLR$new(obj_dml_data, ml_g, ml_m)
par_grids = list("ml_g" = paradox::ParamSet$new(list(
    paradox::ParamInt$new("num.trees", lower = 5, upper = 6, default = 5))),
    "ml_m" = paradox::ParamSet$new(list(
    paradox::ParamInt$new("num.trees", lower = 5, upper = 6, default = 5))))
tune_settings = list(
    n_folds_tune = 5,
    rsmp_tune = mlr3::rsmp("cv", folds = 5),
    measure = NULL,
    terminator = mlr3tuning::trm("evals", n_evals = 20),
    algorithm = mlr3tuning::tnr("grid_search"),
    resolution = 5)

dml_plr_obj$tune(param_set=par_grids, tune_settings = tune_settings)

Expected Result

No exception

Actual Result

Exception

Error in private$assert_tune_settings(tune_settings) : 
  Assertion on 'tune_settings$measure' failed: Must be of type 'list', not 'NULL'. 

Versions

> sessionInfo()
R version 4.0.4 (2021-02-15)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 21.10

> packageVersion('DoubleML')
[1] ‘0.4.1’
> packageVersion('mlr3')
[1] ‘0.11.0.9000’