Prevent crash due to null Parent reference
Prevent crash due to null Parent reference

- Unwire event listener when parent is null
- Issue MatterHackers/MCCentral#4357
Crash due to "Add Content" -> popup logic problems

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