Action "Edit" triggers an error in an empty project when no item is currently selected (e.g. empty planning)

[vld-codra]

Create a new planning, then click Edit in a context menu/toolbar.

Exception raised:

>>> Traceback (most recent call last):
  File "[...]\guidata\qthelpers.py", line 169, in <lambda>
    action.triggered.connect(lambda checked=False: triggered())
                                                   ^^^^^^^^^^^
  File "[...]\PyPlanning\planning\gui\treewidgets.py", line 470, in edit_current_item
    self.edit(item.index())
              ^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'index'