Temp Fix...
Piggybacking off the plugin suggestions, without installing another plugin, when you encounter the unselectable model error, you can go into "Ultimaker > About Ultimaker Cura" in the menu on the mac, which opens a popup. Close it and now your model is selectable and you lose none of your edits, supports, etc.
Hope that helps. I've lost so much work in the past. Working on a model for half an hour adding supports, and then having to restart. No bueno. Hope this helps you guys! Take care.