Class for user actions
Extract your preferences code into extra class or create extension functions to get those objects. Ex. sharedPreferences.loadObject(GlobalConstants.OBJECTIVE_SAVE_KEY + user.userName,Objective::class.java) should have it's own method. Ex. loadUserByName.
You are using kotlin, use it's strengths like non-nullables by default. Does repository.updateUserName() really accept nullables? That seems wrong. I'd use it for example with elvis operator loadUserByName(user.username) ?: error("username not found")