Fix PySide error when ViewBox signal destroyed
Fix PySide error when ViewBox signal destroyed

Fixes issue #107

onlyjus/pyqtgraph,mpvismer/pyqtgraph,welshjf/pyqtgraph,ddamiani/pyqtgraph,mgraupe/acq4,tropp/acq4,benvermaercke/pyqtgraph,mgraupe/acq4,tropp/acq4,pbmanis/acq4,nmearl/pyqtgraph,pbmanis/acq4,tropp/acq4,gvalentine/pyqtgraph,ericdill/pyqtgraph,campagnola/acq4,gvalentine/pyqtgraph,petebachant/pyqtgraph,mgraupe/acq4,ddamiani/pyqtgraph,Lx37/pyqtgraph,benvermaercke/pyqtgraph,jlombacher/pyqtgraph,ng110/pyqtgraph,jlombacher/pyqtgraph,carlomt/dicom_tools,tropp/acq4,meganbkratz/acq4,acq4/acq4,ng110/pyqtgraph,derricw/pyqtgraph,duguxy/pyqtgraph,ericdill/pyqtgraph,petebachant/pyqtgraph,stscieisenhamer/pyqtgraph,gvalentine/pyqtgraph,meganbkratz/acq4,pmaunz/pyqtgraph,duguxy/pyqtgraph,campagnola/acq4,JenSte/pyqtgraph,welshjf/pyqtgraph,benvermaercke/pyqtgraph,mpvismer/pyqtgraph,mgraupe/acq4,campagnola/acq4,acq4/acq4,mgraupe/acq4,pbmanis/acq4,welshjf/pyqtgraph,nmearl/pyqtgraph,onlyjus/pyqtgraph,campagnola/acq4,carlomt/dicom_tools,onlyjus/pyqtgraph,ddamiani/pyqtgraph,meganbkratz/acq4,pbmanis/acq4,meganbkratz/acq4,acq4/acq4,derricw/pyqtgraph,acq4/acq4,ericdill/pyqtgraph,JenSte/pyqtgraph,pmaunz/pyqtgraph,Lx37/pyqtgraph,stscieisenhamer/pyqtgraph