Fix crash if man is not installed when hit '?' and 'm'.

On some systems (e.g. archlinuxarm) man is not installed by default, hit
'?' and 'm' will cause ranger crashed, showing:
AttributeError: 'NoneType' object has no attribute 'poll'.

1 files changed, 3 insertions, 0 deletions

diff --git a/ranger/core/actions.py b/ranger/core/actions.py
index adfbe94d..c458da11 100644
--- a/ranger/core/actions.py
+++ b/ranger/core/actions.py
@@ -974,6 +974,9 @@ class Actions(  # pylint: disable=too-many-instance-attributes,too-many-public-m
             if process.poll() != 16:
                 return
         process = self.run(['man', 'ranger'])
+        if process is None:
+            self.notify("Show manpage failed, check if man is installed", bad=True)
+            return
         if process.poll() == 16:
             self.notify("Could not find manpage.", bad=True)