"Error in UseMethod("xml_add_child") " test_dir doesn't work

When I try to define a path in test_dir function it says that error:

Error in UseMethod("xml_add_child") : no applicable method for 'xml_add_child' applied to an object of class "NULL"

My testthat.R file has this lines:

library(testthat)
library(package)

options(testthat.junit.output_file = "test-out.xml")
test_dir("tests/testthat/0_utils/", reporter = 'junit')
test_package("package")

My tests are in 'tests/testthat/0_utils' folder. How can i solve this problem?