Issue: Your crawl component stops responding and the content source is is stuck on a crawling, starting, or stopping status.  You may try a multitude of solutions and eventually attempt to delete and recreate the service application.  Upon creation of the search service application and opening it you are greeted by this message.

Resolution:  This error is a fun one, and by fun I mean painful.  I researched a lot of various solutions but the most consistent answer was as follows.  Please note that this is not a guaranteed resolution, but a common one:

1. Delete your search applications
2. Run the Farm Configuration Wizard to create a search service application
3. Rec-create your search service applications as needed
1. Note: you may receive the same error when initially created.  Give it a few minutes.