Xcode: issue "file xxx.png is missing from working copy" at project building

After deleting/adding some png files to project, i have got messages when building project.

"file ProjectPath\aaa\xxx.png is missing from working copy."

All these files are in the project, and the application is running. However, these messages are annoying. Looked .plist file, but there is no mention of these files. What should I do to remove these messages?

The warning will disappear as soon as you commit your changes (Xcode 8).

It seems that this problem may have different causes, but it's often in relation with source control software.

In my case, I solved it by going to Git, and adding the files again. I mean running the following command:

git add .