FUEL: do not fail if there is no file associated with current buffer

misc/fuel/fuel-mode.el

@@ -174,8 +174,11 @@ interacting with a factor listener is at your disposal.
   (setq fuel-stack-mode-string "/S")
   (when fuel-mode-stack-p (fuel-stack-mode fuel-mode))
 
-  (when (and fuel-mode (not (file-exists-p (buffer-file-name))))
-    (fuel-scaffold--maybe-insert)))
+  (let ((file-name (buffer-file-name)))
+    (when (and fuel-mode
+               file-name
+               (not (file-exists-p file-name)))
+      (fuel-scaffold--maybe-insert))))
 
 
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