Fix warning on application exit.
Fix warning on application exit.

Was getting the following warning on app exit on Linux

   QCoreApplication::postEvent: Unexpected null receiver

Fixed that.

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