Steps to repro:
1. yy on first file
2. ya on second file
3. Then I tried run: ":shell vimdiff %c"

BUG - I didn't do anything, and after pressing 2nd or 3rd time Enter key, ranger crashed.

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ranger/core/main.py", line 133, in main
    fm.loop()
  File "/usr/lib/python2.7/site-packages/ranger/core/fm.py", line 314, in loop
    ui.handle_input()
  File "/usr/lib/python2.7/site-packages/ranger/gui/ui.py", line 208, in handle_input
    self.handle_key(key)
  File "/usr/lib/python2.7/site-packages/ranger/gui/ui.py", line 144, in handle_key
    self.press(key)
  File "/usr/lib/python2.7/site-packages/ranger/gui/ui.py", line 159, in press
    quantifier=keybuffer.quantifier)
  File "/usr/lib/python2.7/site-packages/ranger/core/actions.py", line 169, in execute_console
    escape=cmd.escape_macros_for_shell)
  File "/usr/lib/python2.7/site-packages/ranger/core/actions.py", line 184, in substitute_macros
    macros = self._get_macros()
  File "/usr/lib/python2.7/site-packages/ranger/core/actions.py", line 243, in _get_macros
    if tabdir.get_selection():
  File "/usr/lib/python2.7/site-packages/ranger/container/directory.py", line 161, in get_selection
    return [item for item in self.files if item.marked]
TypeError: 'NoneType' object is not iterable

OS: Fedora 22
ranger-stable 1.6.1
Python 2.7.10 (default, Jun 20 2016, 14:45:40)
[GCC 5.3.1 20160406 (Red Hat 5.3.1-6)]