Applications using Launch Screen Files and targetting iOS 7.1 and earlier need to also include a Launch Image in an Asset Catalog.
I ran into the exact same problem this morning. Here's how I fixed it.
1、Select your project in the project navigator. Under the General tab, you should see the section for App Icons and Launch Images. Where you see Launch Images Source, click 'Use Asset Catalog'.
2、You will then see Migrate launch images to an asset catalog dialogue. Click 'Migrate'.
3、Make sure that your Launch Images Source on the previous screen points to the right image set in your asset catalog.In your case, you would set it to 'Default'.
That should take care of the warning. Hope that helps!