Unknown Class **** in Interface

Sometimes IBuilder missed customModule="AppName" customModuleProvider="target"

To fix it, open storyboard as source code and replace this line:

<viewController storyboardIdentifier="StoryboardId" id="SomeID" customClass="CustomClass"
sceneMemberID="viewController">
to this:

<viewController storyboardIdentifier="StoryboardId" id="SomeID" customClass="CustomClass"
customModule="AppName" customModuleProvider="target" sceneMemberID="viewController">