Xcode 12 - SwiftUI preview doesn't work on Swift Package when have another Swift Package as dependencies - 'previewInstances' message to agent

Workaround for both iOS and macOS (not tested with Catalyst)

public let imageBundle = Bundle.myModuleprivate class CurrentBundleFinder {}extension Foundation.Bundle {    static var myModule: Bundle = {        /* The name of your local package, prepended by "LocalPackages_" for iOS and "PackageName_" for macOS. You may have same PackageName and TargetName*/        let bundleNameIOS = "LocalPackages_TargetName"        let bundleNameMacOs = "PackageName_TargetName"        let candidates = [            /* Bundle should be present here when the package is linked into an App. */            Bundle.main.resourceURL,            /* Bundle should be present here when the package is linked into a framework. */            Bundle(for: CurrentBundleFinder.self).resourceURL,            /* For command-line tools. */            Bundle.main.bundleURL,            /* Bundle should be present here when running previews from a different package (this is the path to "…/Debug-iphonesimulator/"). */            Bundle(for: CurrentBundleFinder.self).resourceURL?.deletingLastPathComponent().deletingLastPathComponent().deletingLastPathComponent(),            Bundle(for: CurrentBundleFinder.self).resourceURL?.deletingLastPathComponent().deletingLastPathComponent(),        ]                for candidate in candidates {            let bundlePathiOS = candidate?.appendingPathComponent(bundleNameIOS + ".bundle")            let bundlePathMacOS = candidate?.appendingPathComponent(bundleNameMacOs + ".bundle")            if let bundle = bundlePathiOS.flatMap(Bundle.init(url:)) {                return bundle            } else if let bundle = bundlePathMacOS.flatMap(Bundle.init(url:)) {                return bundle            }        }        fatalError("unable to find bundle")    }()}

Update 1:

Sorry my last post was not accurate. This is a bug in Xcode with no work around. Just submitted to Apple a bug report (FB8880328). Also, posted a details write up with example code w/ repro steps here. Direct link to GitHub project: https://github.com/ryanholden8/SwiftUI-Preview-Failing-Test-Project

Old Post:

Got this exact error doing the same thing, putting colors in a separate package. This post helped me get to the bottom of it. I deleted the default class that is generated in the colors package. However, I did not delete the unit test that was based on that default class.

In short: Delete the auto-generated unit test in your Common package. Or make sure all the unit tests pass.