MRView: fix segfault when using Screen Capture tool
MRView: fix segfault when using Screen Capture tool

Closes issue #190

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3