Unregister Scene_SelectionChanged on close
Unregister Scene_SelectionChanged on close

- Issue MatterHackers/MCCentral#2113
Null reference error after changing theme colors

jlewin/MatterControl,larsbrubaker/MatterControl,mmoening/MatterControl,larsbrubaker/MatterControl,jlewin/MatterControl,larsbrubaker/MatterControl,mmoening/MatterControl,unlimitedbacon/MatterControl,unlimitedbacon/MatterControl,unlimitedbacon/MatterControl,jlewin/MatterControl,jlewin/MatterControl,mmoening/MatterControl,larsbrubaker/MatterControl,unlimitedbacon/MatterControl