Bug: Effort prompt write malformed input in agenda file even after reporting the error [9.1.9 (release_9.1.9-65-g5e4542 @ /usr/local/share/emacs/26.3/lisp/org/)]

[Firmin Martin]

Remember to cover the basics, that is, what you expected to happen and
what in fact did happen.  You don't know how to make a good report?  See

     https://orgmode.org/manual/Feedback.html#Feedback

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In the agenda buffer, I've called the `org-set-effort` function
with the `e` key, and input "-" which is malformed. I did receive
an error `Invalid duration format: "-"`, but the "-" has still been
written in the entry as an effort property.

As a consequence of this bug, opening the agenda buffer raise again
the error `Invalid duration format: "-"` which results that the agenda
buffer cannot be opened. The malformed line  

     :Effort:   -

can only be found by a dichotomic search which is quite annoying.
For a similar issue, see here

https://emacs.stackexchange.com/questions/36111/invalid-duration-format-org-agenda/53637#53637

Maybe other functions have the same behavior (writing
malformed value on the agenda even after reporting the error).


Emacs  : GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.8)
 of 2019-10-22
Package: Org mode version 9.1.9 (release_9.1.9-65-g5e4542 @ 
/usr/local/share/emacs/26.3/lisp/org/)