error when file manager can't be determined

[groovy]

When right-clicking on a thumbnail and selecting "Open in File Browser" I get the following error:

DEBUG Launching: None file:///run/media/groovy/NIKON%20D90/DCIM/100NCD90/_DSC5555.NEF
ERROR An unhandled exception occurred
ERROR Traceback (most recent call last):
  File "/usr/lib64/python3.4/site-packages/raphodo/thumbnaildisplay.py", line 1479, in doOpenInFileBrowserAct
    subprocess.Popen(args)
  File "/usr/lib64/python3.4/subprocess.py", line 859, in __init__
    restore_signals, start_new_session)
  File "/usr/lib64/python3.4/subprocess.py", line 1463, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] Datei oder Verzeichnis nicht gefunden: 'None'

On the console, running in debug-mode, I see:

DEBUG Default file manager could not be determined

Version information:

Rapid Photo Downloader: 0.9.0a4
Platform: Linux-4.6.3-gentoo-x86_64-Intel-R-_Core-TM-_i7_CPU_M_640_@_2.80GHz-with-gentoo-2.2
Python: 3.4.4
Qt: 5.6.0
PyQt: 5.6.1.dev1604271126
ZeroMQ: 4.1.4
Python ZeroMQ: 15.2.0
gPhoto2: 2.5.10
Python gPhoto2: 1.4.0
ExifTool: 10.20
pymediainfo: 2.1.4
GExiv2: 0.10.03
psutil: 4.1.0
Exiv2: 0.25.0