“Plain Style unsupported in a Navigation Item” warning with my customized Bar Button Item

Question

I drag a Round Rect Button to the position of the right Bar Button Item, and set an image to the Round Rect Button. All works well, except the warning \"Plain Style unsuppor

Answer 1

In my case it was an image of a back arrow, which was set to "Plain".

The error message appears to refer to all the items in the navigation bar.