ScrollArea segfaults in an special case

[helios2000]

The backtrace shows a segfault that happens during the destruction of a ScrollArea. In Widget::clear() it fails to call _getVisibilityEventHandler(). The reason for that is, it happens after the main gui is destroyed.

Related UH issue, unknown-horizons/unknown-horizons#2604

#0 0x0000000000000000 in ?? ()
#1 0x00007f481f302030 in fcn::Widget::clear() () from /usr/lib/x86_64-linux-gnu/libfifechan.so.0.1.3
#2 0x00007f481f30c60e in fcn::ScrollArea::setContent(fcn::Widget*) () from /usr/lib/x86_64-linux-gnu/libfifechan.so.0.1.3
#3 0x00007f481f30c833 in fcn::ScrollArea::~ScrollArea() () from /usr/lib/x86_64-linux-gnu/libfifechan.so.0.1.3
#4 0x00007f481f30c859 in fcn::ScrollArea::~ScrollArea() () from /usr/lib/x86_64-linux-gnu/libfifechan.so.0.1.3
#5 0x00007f4812171fda in _wrap_delete_ScrollArea (args=)
at ./obj-x86_64-linux-gnu/fifechanPYTHON_wrap.cxx:9035
#6 0x00005646a73d15f3 in PyObject_Call ()
#7 0x00005646a73da261 in PyObject_CallFunctionObjArgs ()
#8 0x00007f481215d450 in SwigPyObject_dealloc (v=0x7f480c8dcd50) at ./obj-x86_64-linux-gnu/fifechanPYTHON_wrap.cxx:1720
#9 0x00005646a741e496 in ?? ()
#10 0x00005646a73c826c in ?? ()
#11 0x00005646a7422181 in PyGC_Collect ()
#12 0x00005646a74215be in Py_Finalize ()
#13 0x00005646a73be954 in Py_Main ()
#14 0x00007f4823ecd2b1 in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
#15 0x00005646a73be3aa in _start ()