fix: guard org-drill-again and org-drill-resume against nil last-session

Both functions bound session to org-drill-last-session and immediately
called setf / org-drill-entries-pending-p on it without checking for
nil.  First-time invocation (or after Emacs restart with no active
session) threw an obscure eieio-oset / nil-slot type error instead
of a clear message.

Added (unless session (user-error ...)) at the top of each function.
A user running M-x org-drill-resume cold now sees a sensible message
telling them to run org-drill first.

2 files changed, 43 insertions, 0 deletions

diff --git a/org-drill.el b/org-drill.el
index 97ba891..0de606c 100644
--- a/org-drill.el
+++ b/org-drill.el
@@ -3231,6 +3231,8 @@ unreviewed items. If there are no leftover items in memory, a full
 scan will be performed."
   (interactive)
   (let ((session org-drill-last-session))
+    (unless session
+      (user-error "No previous drill session to continue — run `org-drill' first"))
     (setf (oref session cram-mode) nil)
     (cond
      ((cl-plusp (org-drill-pending-entry-count session))
@@ -3249,6 +3251,8 @@ scan will be performed."
 exiting them with the `edit' or `quit' options."
   (interactive)
   (let ((session org-drill-last-session))
+    (unless session
+      (user-error "No suspended drill session to resume — run `org-drill' first"))
     (cond
      ((org-drill-entries-pending-p session)
       (org-drill nil nil t))
diff --git a/tests/test-org-drill-resume-nil-session.el b/tests/test-org-drill-resume-nil-session.el
new file mode 100644
index 0000000..fd64faf
--- /dev/null
+++ b/tests/test-org-drill-resume-nil-session.el
@@ -0,0 +1,39 @@
+;;; test-org-drill-resume-nil-session.el --- Regression for nil last-session  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `org-drill-again' and `org-drill-resume' bind a local `session' to
+;; `org-drill-last-session' without checking for nil.  On the first
+;; ever invocation (or after Emacs restart with no active session),
+;; org-drill-last-session is nil and both functions throw — `again'
+;; on `(setf (oref session cram-mode) nil)' (eieio-oset on nil),
+;; `resume' on `org-drill-entries-pending-p' walking nil's slots.
+;;
+;; Fixed by adding an early `(unless session (user-error ...))' to
+;; both.  A clear user-visible message replaces the obscure type
+;; errors.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(require 'org)
+(require 'org-drill)
+
+;;;; Regression — org-drill-again
+
+(ert-deftest test-org-drill-again-nil-last-session-raises-user-error ()
+  "Calling org-drill-again with no prior session triggers a user-error,
+not an obscure eieio type error."
+  (let ((org-drill-last-session nil))
+    (should-error (org-drill-again) :type 'user-error)))
+
+;;;; Regression — org-drill-resume
+
+(ert-deftest test-org-drill-resume-nil-last-session-raises-user-error ()
+  "Calling org-drill-resume with no prior session triggers a user-error."
+  (let ((org-drill-last-session nil))
+    (should-error (org-drill-resume) :type 'user-error)))
+
+(provide 'test-org-drill-resume-nil-session)
+
+;;; test-org-drill-resume-nil-session.el ends here