Complicated Runtime error

When I open yade it looks like the following: I am attempting to run the code on the site that is called bouncing sphere. I press F12-Load-search-my python file with the named ball.py and after I open it, the terminal has a runtime error. When I go to the generate tab on my controller I am able to press generate and there is a simulation of balls compacting. I don't know why I am not able to run the bouncing sphere example

vincia@ubuntu:~$ yade
Welcome to Yade 0.60.3
TCP python prompt on localhost:9000, auth cookie `ssuade'
XMLRPC info provider on http://localhost:21000

(python:2650): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",

(python:2650): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",

(python:2650): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",

(python:2650): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",
[[ ^L clears screen, ^U kills line. F12 controller, F11 3d view, F10 both, F9 generator, F8 plot. ]]
Yade [1]: yade.qt.Controller();
Yade [2]: (python:2650): Gtk-DEBUG: No tracker backend available
INFO /build/buildd/yade-0.60.3/core/Omega.cpp:222 loadSimulation: Loading file /home/vincia/yade/examples/ball.py
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)

/usr/lib/yade/py/yade/__init__.pyc in loadSlot(self)
    138 if not f: return # cancelled
    139 self.deactivateControls()
--> 140 O.load(f)
    141 def saveSlot(self):
    142 f=QFileDialog.getSaveFileName(self,'Save simulation','','Yade simulations (*.xml *.xml.bz2 *.xml.gz *.yade *.yade.gz *.yade.bz2);; *.*')

RuntimeError: invalid signature