Does this happen on all renderings? or specific ones?
I would go ahead and check to ensure the rendering (controller type) has correct action and settings on the item itself, secondly may be the error handling is not done right on your controller method, what would happen when datasource is not set yet? Are you able and are confident that you are able to pick the correct datasource?
From the looks of it, it is not able to find the correct controller. Ensure your code is built and items corresponding to controller are existing.
To isolate the problem, try adding a view rendering by defining one and adding it to experience editor.