Toolbox woes

I am trying to write a toolbox program. It displays an autocreated window with a writable field. When I try to enter text into it, the program crashes with Fatal signal received: segmentation fault . Yet earlier on when I was using WritableField_ValueChanged the program was not crashing. It is a long shot, but I wondered if others might have encountered this particular dragon during their quests into the dark realms.

I can persuade a dragon of sorts to appear, which may not be the same dragon.

An AppBasic app that auto-shows an auto-created Window containing a Writable field of ‘null’ length errors on attempting to enter text with :-

BasicApp: Internal error: abort on data transfer at &FC141FC8 (code28)

*where
Address &FC141FD0 is at offset &00012D74 in module 'WindowManager'

The starting condition for the Writable icon is nonsense, and accidental, but none the less ResEd allows it.

In ResEd Writable field Text is empty but Length is *.

A demo is here

Tested on a Titanium, OS 5.29 (14-Sep-21).

Wasn’t there something recently where ResEd was emitting non-‘null’, but empty validation strings, which aren’t valid (see the BNF in the PRM)? Also note that ‘null’ for validation strings should in fact be -1 (though the only place this seems specified is in the PRM Title Data window block, saying “It has the same interpretation as the icon data bytes described under Wimp_CreateIcon”)?

[Edit: what I was thinking of appeared to be the ccres tool, see Bugs #519, but maybe the diff strings came originally from ResEd.]