Here's how I fixed it... Select your Extension target and go to Build Settings Search for module and look for Packaging -> Product Module Name. You'll probably see that the Resolved column is showing the same name as your main target. Type a unique name in the middle column (the column is titled WidgetsExtension or whatever your extension target is called) You should see that the Resolved column now shows the unique name you entered. It should hopefully build now.
Topic:
App & System Services
SubTopic:
Core OS
Tags: