Since updating Ember from 2.7.3 to 2.9.1*, the component does not render anymore, and my surrounding component's test fails, with the above error message:
Error: Property set failed: object in path "targetObject" could not be found or was destroyed.
at EmberError (http://localhost:4200/assets/vendor.js:xxxxx:xx
Commenting out the line this.set('targetObject.' + this.get('register-as'), this); fixes the issue for me, but I so far have no need "to be able to send actions from outside" (as the line is explained in a comment above), and I am unsure how this would be handled otherwise (this.get('parentController')+'.' instead of targetObject. does not change the error).