MRView: fix crash when opening View tool without an image in main viewer
MRView: fix crash when opening View tool without an image in main viewer

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