Description of problem:
gedit folding plugin reports an unhandled exception.

Version-Release number of selected component (if applicable):
3.18.2

How reproducible:
Always

Steps to Reproduce:
1. execute gnome-text-editor
2. 
3.

Actual results:
Traceback (most recent call last):
  File "/home/jprajzne/.local/share/gedit/plugins/gedit-folding/folding.py", line 29, in do_activate
    self._insert_menu()
  File "/home/jprajzne/.local/share/gedit/plugins/gedit-folding/folding.py", line 53, in _insert_menu
    manager = self.window.get_ui_manager()
AttributeError: 'Window' object has no attribute 'get_ui_manager'
Traceback (most recent call last):
  File "/home/jprajzne/.local/share/gedit/plugins/gedit-folding/folding.py", line 38, in do_update_state
    action_group = self._action_group
AttributeError: 'FoldingPyPlugin' object has no attribute '_action_group'
Traceback (most recent call last):
  File "/home/jprajzne/.local/share/gedit/plugins/gedit-folding/folding.py", line 38, in do_update_state
    action_group = self._action_group
AttributeError: 'FoldingPyPlugin' object has no attribute '_action_group'
Traceback (most recent call last):
  File "/home/jprajzne/.local/share/gedit/plugins/gedit-folding/folding.py", line 38, in do_update_state
    action_group = self._action_group
AttributeError: 'FoldingPyPlugin' object has no attribute '_action_group'
Traceback (most recent call last):
  File "/home/jprajzne/.local/share/gedit/plugins/gedit-folding/folding.py", line 38, in do_update_state
    action_group = self._action_group
AttributeError: 'FoldingPyPlugin' object has no attribute '_action_group'

gedit[18402]: detected unhandled Python exception in 'gedit' in journal

Expected results:
no errors

Additional info:
The "AttributeError: 'FoldingPyPlugin' object has no attribute '_action_group'" is reported every time when opening a new tab in gedit.