Add missing initialization of member variable
Add missing initialization of member variable

m_mainWindow was not initialized at the time the transform menu was
being built, leading to a crash in CropReaction when the
EditOperatorDialog was instantiated.

OpenChemistry/tomviz,thewtex/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,cjh1/tomviz,cryos/tomviz,cryos/tomviz,cjh1/tomviz,mathturtle/tomviz,thewtex/tomviz,thewtex/tomviz,mathturtle/tomviz,cryos/tomviz,cjh1/tomviz,OpenChemistry/tomviz,mathturtle/tomviz