8.3 hangs

[Samuel Wales <samologist@gmail.com>]

ret and tab and sort hang emacs.  previously i got Error
running timer `org-element--cache-sync':
(wrong-type-argument integer-or-marker-p nil).

tdoq at hang produces this (not running on source only):

Debugger entered--Lisp error: (quit)
  org-element--cache-compare...
  avl-tree--do-delete(org-element--cache-compare...
  avl-tree--do-delete(org-element--cache-compare...
  avl-tree--do-delete(org-element--cache-compare...
  avl-tree--do-delete(org-element--cache-compare...
  avl-tree-delete([cl-struct-avl-tree-...
  ...
  org-element--cache-process-request([30317 37858 46912 -21480 (drawer
(:begin 46764 :end 46912 :drawer-name "LOGBOOK" :contents-begin 46776
:contents-end 46900 :post-blank 0 :post-affiliated 46764 :parent nil))
0] 55567 1 nil nil)
  ...
  org-element--cache-sync(#<buffer executive--a.org> 1)
  org-element-at-point()
  org-hide-block-toggle()
  org-hide-block-toggle-maybe()
  run-hook-with-args-until-success(org-hide-block-toggle-maybe)
  org-cycle(nil)
  call-interactively(org-cycle nil nil)
  command-execute(org-cycle)

cache reset sometimes fixes it for a while.