gist export broken

Created by: zeffii

@Durman

Error: Python: Traceback (most recent call last):
  File "C:\Users\zeffi\Desktop\scripts\addons_contrib\sverchok\ui\sv_IO_panel.py", line 299, in execute
    is_tree_exportable, msg = self.can_be_exported(ng)
  File "C:\Users\zeffi\Downloads\blender29\2.93\scripts\modules\bpy_types.py", line 720, in __getattribute__
    return super().__getattribute__(attr)
AttributeError: 'SvNodeTreeExportToGist' object has no attribute 'can_be_exported'

location: <unknown location>:-1

where is can_be_exported defined.

Originally posted by @zeffii in https://github.com/nortikin/sverchok/issues/4149#issuecomment-856194760