Bug in editable trees (Desktop Client)
A bug was detected in editable trees like "Timesheet Lines" (Timesheet module).
This bug was present in Tryton 4, and it remained in Tryton 5.
Steps to reproduce:
1) Open a tab with an editable tree with more than one rows.
2) Modify any field in any row.
3) Press the Page Down key (or Page Up key).
4) Answer "No" on question "...do you want to save it?".
5) The following application error occurs:
`File "C:\msys32\home\ced\5.2\tryton/tryton/gui/window/form.py", line 400, in sig_next`
`File "C:\msys32\home\ced\5.2\tryton/tryton/gui/window/view_form/screen/screen.py", line 914, in display_next`
`TypeError: cannot unpack non-iterable NoneType object`
P.S. If in step 4 you answer “Yes” or “Cancel”, then everything is OK.