org-clock.el (org-clock-select-task): Throw a user error when the clock history is empty

* org-clock.el (org-clock-select-task): Throw a user error
when the clock history is empty.

lisp/org-clock.el

@@ -514,47 +514,51 @@ of a different task.")
   "Hook called in task selection just before prompting the user.")
 
 (defun org-clock-select-task (&optional prompt)
-  "Select a task that recently was associated with clocking."
+  "Select a task that was recently associated with clocking."
   (interactive)
-  (let (sel-list rpl (i 0) s)
-    (save-window-excursion
-      (org-switch-to-buffer-other-window
-       (get-buffer-create "*Clock Task Select*"))
-      (erase-buffer)
-      (when (marker-buffer org-clock-default-task)
-	(insert (org-add-props "Default Task\n" nil 'face 'bold))
-	(setq s (org-clock-insert-selection-line ?d org-clock-default-task))
-	(push s sel-list))
-      (when (marker-buffer org-clock-interrupted-task)
-	(insert (org-add-props "The task interrupted by starting the last one\n" nil 'face 'bold))
-	(setq s (org-clock-insert-selection-line ?i org-clock-interrupted-task))
-	(push s sel-list))
-      (when (org-clocking-p)
-	(insert (org-add-props "Current Clocking Task\n" nil 'face 'bold))
-	(setq s (org-clock-insert-selection-line ?c org-clock-marker))
-	(push s sel-list))
-      (insert (org-add-props "Recent Tasks\n" nil 'face 'bold))
-      (mapc
-       (lambda (m)
-	 (when (marker-buffer m)
-	   (setq i (1+ i)
-		 s (org-clock-insert-selection-line
-		    (if (< i 10)
-			(+ i ?0)
-		      (+ i (- ?A 10))) m))
-	   (if (fboundp 'int-to-char) (setf (car s) (int-to-char (car s))))
-	   (push s sel-list)))
-       org-clock-history)
-      (run-hooks 'org-clock-before-select-task-hook)
-      (goto-char (point-min))
-      (org-fit-window-to-buffer nil nil (+ 6 (length org-clock-history)))
-      (message (or prompt "Select task for clocking:"))
-      (setq cursor-type nil rpl (read-char-exclusive))
-      (cond
-       ((eq rpl ?q) nil)
-       ((eq rpl ?x) nil)
-       ((assoc rpl sel-list) (cdr (assoc rpl sel-list)))
-       (t (error "Invalid task choice %c" rpl))))))
+  (let ((chl (length org-clock-history)) sel-list rpl (i 0) s)
+    (if (zerop chl)
+	(user-error "No recent clock")
+      (save-window-excursion
+	(org-switch-to-buffer-other-window
+	 (get-buffer-create "*Clock Task Select*"))
+	(erase-buffer)
+	(when (marker-buffer org-clock-default-task)
+	  (insert (org-add-props "Default Task\n" nil 'face 'bold))
+	  (setq s (org-clock-insert-selection-line ?d org-clock-default-task))
+	  (push s sel-list))
+	(when (marker-buffer org-clock-interrupted-task)
+	  (insert (org-add-props "The task interrupted by starting the last one\n" nil 'face 'bold))
+	  (setq s (org-clock-insert-selection-line ?i org-clock-interrupted-task))
+	  (push s sel-list))
+	(when (org-clocking-p)
+	  (insert (org-add-props "Current Clocking Task\n" nil 'face 'bold))
+	  (setq s (org-clock-insert-selection-line ?c org-clock-marker))
+	  (push s sel-list))
+	(insert (org-add-props "Recent Tasks\n" nil 'face 'bold))
+	(mapc
+	 (lambda (m)
+	   (when (marker-buffer m)
+	     (setq i (1+ i)
+		   s (org-clock-insert-selection-line
+		      (if (< i 10)
+			  (+ i ?0)
+			(+ i (- ?A 10))) m))
+	     (if (fboundp 'int-to-char) (setf (car s) (int-to-char (car s))))
+	     (push s sel-list)))
+	 org-clock-history)
+	(run-hooks 'org-clock-before-select-task-hook)
+	(goto-char (point-min))
+	;; Set min-height relatively to circumvent a possible but in
+	;; `fit-window-to-buffer'
+	(fit-window-to-buffer nil nil (if (< chl 10) chl (+ 5 chl)))
+	(message (or prompt "Select task for clocking:"))
+	(setq cursor-type nil rpl (read-char-exclusive))
+	(cond
+	 ((eq rpl ?q) nil)
+	 ((eq rpl ?x) nil)
+	 ((assoc rpl sel-list) (cdr (assoc rpl sel-list)))
+	 (t (user-error "Invalid task choice %c" rpl)))))))
 
 (defun org-clock-insert-selection-line (i marker)
   "Insert a line for the clock selection menu.