Comment 2 for bug 850080

StacktraceTop:
 __GI___assert_fail (assertion=0x7fe1cabd6bcc "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-4.14.2/plugins/unityshell/src/unityshell.cpp:2247
 glPaintOutput (this=<optimized out>, attrib=<optimized out>, transform=<optimized out>, region=<optimized out>, output=<optimized out>, mask=<optimized out>) at /build/buildd/unity-4.14.2/plugins/unityshell/src/unityshell.cpp:750
 UnityScreen::glPaintOutput (this=0x272d6d0, attrib=..., transform=..., region=..., output=0x509bfd0, mask=3) at /build/buildd/unity-4.14.2/plugins/unityshell/src/unityshell.cpp:741