I'm trying to build a Mac Catalyst version of our iOS (iPad) app but the build always fails with 'Internal error, please file a bug at ...' when compiling MainWindow.xib file. (I have already filed a bug.)
The error in the log.txt file is: "Failed to compile nib consisting of IBProxyObject (x2), IBUICustomObject (x1), IBUINavigationBar (x1), IBUINavigationController (x1), IBUINavigationItem (x1), IBUIToolbar (x1), IBUIViewController (x1), and IBUIWindow (x1).
Exception name: IBPlatformToolFailureException Exception reason: The tool closed the connection (IBAgent-macOS-UIKit)"
The same .xib file works fine when building an iOS version of the app and it worked for MAC catalyst until Xcode v26.x. I've already tried cleaning the build folder, deleted project DerivedData folder, replaced all custom classes with base classes and tried other suggestions found in the internet but nothing seems to help.