The editor saves your rig by itself
Once your creature has a skeleton, every change is saved a few seconds later, and again when you leave.
As soon as your creature has a skeleton, the editor saves each change a few seconds after you make it, and once more when you go back to the gallery or close the tab. The Save button shows ✓ Autosaved.
You no longer lose a finished rig because you closed the editor without pressing Save.