Comment 2 for bug 838128

StacktraceTop:
 __GI___assert_fail (assertion=0x7f8ed46b690e "px != 0", file=<optimized out>, line=418, function=<optimized out>) at assert.c:81
 operator-> (this=<optimized out>) at /usr/include/boost/smart_ptr/shared_ptr.hpp:418
 operator-> (this=<optimized out>) at /build/buildd/unity-4.10.2/plugins/unityshell/src/unityshell.cpp:2142
 glPaintOutput (this=<optimized out>, attrib=<optimized out>, transform=<optimized out>, region=<optimized out>, output=<optimized out>, mask=<optimized out>) at /build/buildd/unity-4.10.2/plugins/unityshell/src/unityshell.cpp:671
 UnityScreen::glPaintOutput (this=0x15be500, attrib=..., transform=..., region=..., output=0x103d300, mask=3) at /build/buildd/unity-4.10.2/plugins/unityshell/src/unityshell.cpp:662