Comment 2 for bug 891518

StacktraceTop:
 __GI___assert_fail (assertion=0x7f4f6acbc725 "px != 0", file=<optimized out>, line=418, function=<optimized out>) at assert.c:81
 boost::shared_ptr<UnityFBO>::operator-> (this=<optimized out>) at /usr/include/boost/smart_ptr/shared_ptr.hpp:418
 ~shared_ptr (this=<optimized out>, __in_chrg=<optimized out>) at /usr/include/boost/smart_ptr/shared_ptr.hpp:168
 operator[] (__k=<synthetic pointer>, this=0x1eb6a70) at /usr/include/c++/4.6/bits/stl_map.h:453
 UnityScreen::glPaintOutput (this=0x1eb6600, attrib=..., transform=..., region=..., output=0x6ef0490, mask=3) at /build/buildd/unity-4.24.0/plugins/unityshell/src/unityshell.cpp:949