Failure test_wallpaper.py

gistfile1.txt

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ERROR: test_change_wallpaper (test_wallpaper.TestWallpaper)
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Traceback (most recent call last):
  File "/home/andreeamatei/gaia-ui-tests/gaiatest/tests/test_wallpaper.py", line 39, in test_change_wallpaper
    self.marionette.tap(display_item)
  File "/usr/local/lib/python2.7/dist-packages/marionette_client-0.5.19-py2.7.egg/marionette/marionette_touch.py", line 30, in tap
    self.execute_script("%s.tap(arguments[0], null, null, null, null, arguments[1]);" % self.library_name, [element, send_all])
  File "/usr/local/lib/python2.7/dist-packages/marionette_client-0.5.19-py2.7.egg/marionette/marionette.py", line 497, in execute_script
    scriptTimeout=script_timeout)
  File "/usr/local/lib/python2.7/dist-packages/marionette_client-0.5.19-py2.7.egg/marionette/marionette.py", line 255, in _send_message
    self._handle_error(response)
  File "/usr/local/lib/python2.7/dist-packages/marionette_client-0.5.19-py2.7.egg/marionette/marionette.py", line 290, in _handle_error
    raise JavascriptException(message=message, status=status, stacktrace=stacktrace)
JavascriptException: TypeError: target is null
stacktrace:
        mouseEvent@chrome://marionette/content/marionette-listener.js:911
        drag@chrome://marionette/content/marionette-listener.js:888
        mousetap@chrome://marionette/content/marionette-listener.js:963
        tap@chrome://marionette/content/marionette-listener.js:716
        __marionetteFunc@chrome://marionette/content/marionette-listener.js:1984
TEST-UNEXPECTED-FAIL | test_wallpaper.py TestWallpaper.test_change_wallpaper | @chrome://marionette/content/marionette-listener.js:1984