If you do:
1) C-h f display-buffer
2) Click on pop-up-frames
3) Click on customize
4) Click on the [back] button in the Help buffer
you get the error:
apply: Symbol's function definition is void: nil
This is because help-xref-stack has been set back to nil by clicking on
customize:
In help-
For example, C-h f buffer-list shows this text:
buffer-list is a built-in function in `C source code'.
(buffer-list &optional frame)
Return a list of all existing live buffers.
If the optional arg frame is a frame, we return the buffer list
in the proper order for that frame: the buffers in
