Here a patch. Sao doesn't have the issue since it selects the line before allowing to click on it which is its standard behaviour also for editable list IIRC.

Scenario

• Linux GTK client version 6.2
• https://demo6.2.tryton.org
• Install sale_ammendment module

• Add user to sale ammendment group

• Create ammendment for a sale line, press 'Validate', to make ammendmends tab on sale visible

• Open related sale, go tab ammendments
• Add a new ammendment
• Save sale
• Pressing "Validate" button in list view of ammendments one2many, raises:

ERROR:tryton.common.common:  File "6.2/tryton/tryton/gui/window/view_form/view/list_gtk/widget.py", line 1305, in button_clicked
    self.view.screen.button(self.attrs)
  File "6.2/tryton/tryton/gui/window/view_form/screen/screen.py", line 1195, in button
    if not self.current_record.save(force_reload=False):
AttributeError: 'NoneType' object has no attribute 'save'

N.B. The windows gtk client in version 5.8 fails even harder: it is killed without any message.