To reproduce I create a drill-tagged headline in an empty file and invoke
M-x org-drill, which fails with error message:
Invalid match tag: ""
Works correctly with Org 9.1.14 but not with 9.2; tested in both cases with the outdated org-drill 2.4.7 bundled with
org-plus-contrib and with
org-drill 2.6.1 from this repository.
Presumably connected with the recent org-get-tags meaning change, but I unfortunately lack emacs-fu to diagnose further.