Problem adding new diagnostics to UM

[ReadingClouds]

I just posted this on the MonSoon Teams but this may be more appropriate. TIA for any help!
Adding new diagnostics
We're having trouble adding a new diagnostic to the UM (vn12.0 at present).
I've followed the instructions in https://code.metoffice.gov.uk/doc/um/latest/um-training/stashmaster.html

I have a branch https://code.metoffice.gov.uk/trac/um/browser/main/branches/dev/peterclark/vn12.0_save_ri with changes to STASHmaster-meta.conf and STASHmaster_A (no code yet to actually output the diags but we haven't got to needing that yet!). I cannot see any issue with either.

I have a working copy of a suite u-dc455 and have added the following to app/um/rose-app.conf

meta=um-atmos/HEAD

STASHMASTER=STASHmaster

and (later)

[file:STASHmaster]
source=fcm:um.xm_br/dev/peterclark/vn12.0_save_ri/rose-meta/um-atmos/HEAD/etc/stash/STASHmaster@126161

I now run rose edit with:

rose edit -M ~/fcm_rep/vn12.0_save_ri/rose-meta

When I try to add a new diagnostic to STASH, rose edit crashes with
Traceback (most recent call last):
File "/common/fcm/rose-2019.01.7/lib/python/rose/config_editor/plugin/um/widget/stash.py", line 640, in _launch_new_diagnostic_window
self._refresh_diagnostic_window
File "/common/fcm/rose-2019.01.7/lib/python/rose/config_editor/plugin/um/widget/stash_add.py", line 120, in init
self.generate_tree_view(is_startup=True)
File "/common/fcm/rose-2019.01.7/lib/python/rose/config_editor/plugin/um/widget/stash_add.py", line 143, in generate_tree_view
self._view.set_model(self.get_tree_model())
File "/common/fcm/rose-2019.01.7/lib/python/rose/config_editor/plugin/um/widget/stash_add.py", line 212, in get_tree_model
self._store.append(parent_iter, row_data)
ValueError: row sequence has wrong length

This obviously suggests a problem with the STASHmaster formatting, but
a) I cannot see what that might be and
b) if I revert to the unchanged STASHmaster (i.e. use rev 126160) I get the same crash.

Can anyone suggest what the problem might be?

[SheriffOfRobinhood]

I would like to add some supplementary description of this problem:

The suite could run without error if the STASH request were added manually by adding an item in the suite's UM rose-app.conf as:

[namelist:umstash_streq(IsecItem_DummySequence)]
dom_name= ...
isec=...
item=...
package=...
tim_name=...
use_name=...

where Isec and Item are the section and item number respectively and DummySequence is a dummy 8-digit code made up manually which do not collide with existing umstash_streq item tags.

The direct trigger of the ValueError seems to be a gtk.TreeStore error when the index of hierarchy is faulty:
https://stackoverflow.com/questions/28856425/add-different-length-columns-to-gtk-treestoretreeview

[ReadingClouds]

I've realised the problem: I forgot the suite I was adding a diag to already had additional diagnostics selected which needed to be added to the new STASHmaster as well.