Guard against non-existent plugins dir.

piku.py

@@ -1386,6 +1386,7 @@ def _get_plugin_commands(path):
     sys_path.append(abspath(path))
 
     cli_commands = []
+    if isdir(path):
         for item in listdir(path):
             module_path = join(path, item)
             if not isdir(module_path):