As the summary describes.

Steps to reproduce:
$ mkdir /tmp/test
$ chmod -x /tmp/test
$ ranger /tmp/test
ranger version: 1.6.1, executed with python 2.7.5
Locale: en_US.UTF-8
Current file: None
Traceback (most recent call last):
  File "/tmp/ranger/ranger/core/main.py", line 139, in main
    fm.loop()
  File "/tmp/ranger/ranger/core/fm.py", line 312, in loop
    ui.redraw()
  File "/tmp/ranger/ranger/gui/ui.py", line 265, in redraw
    self.draw()
  File "/tmp/ranger/ranger/gui/ui.py", line 293, in draw
    cwd = self.fm.thisdir.path
AttributeError: 'NoneType' object has no attribute 'path'

An error message explaining the situation would be more useful and user-friendly.