To fix the issue with missing namespace declarations in dependency projects, add the following code to your android/build.gradle:

subprojects {
    afterEvaluate { project ->
        if (project.hasProperty('android')) {
            project.android {
                if (namespace == null) {
                    namespace project.group
                }
            }
        }
    }
}

Note: Make sure that any afterEvaluate call comes before this code, like so:

subprojects {
    project.evaluationDependsOn(":app")
}