pixelated opens in a white window

Neither http://localhost:9090/ is responding:

[2016-05-16 16:45:46] CRITICAL - L#109  : twisted.logger._stdlib:__call__ - Unhandled Error
Traceback (most recent call last):
  File "leap/bitmask/backend_app.py", line 94, in run_backend
    
  File "leap/bitmask/backend/backend.py", line 173, in run
    
  File "site-packages/twisted/internet/base.py", line 1194, in run
    
  File "site-packages/twisted/internet/base.py", line 1203, in mainLoop
    
---  ---
  File "site-packages/twisted/internet/base.py", line 798, in runUntilCurrent
    
  File "leap/bitmask/pix.py", line 50, in start_pixelated_user_agent
    
  File "leap/bitmask/pix.py", line 153, in __init__
    
exceptions.AssertionError: 

I attach the full log.

Restarting bitmask solves the issue.

(from redmine: created on 2016-05-16, relates #8039 (closed))