Android Unit Testing using Mockito for View Model

Question

I have a ViewModel class which has a init block function(s), each function will be observing for a MutableStateFlow<State>, State is nothing but a sealed class, in which i will be updating the value as sucees, Failure (or) loading. Now i need to create a mock View Model but in my case i am having the init block which has the mutablestate that is being observed, now every time i create mock for view model i am getting as null which not expected for a non-null state value, below is my code, i need to know how we can mock the mutablestate observable and write unit test case for the data in the block.

public class ViewModel(
private val observeJoinDataUseCase: ObserveJoinDataUseCase
) {

init{
subscribeOnJoinData()
}

fun subscribeOnJoinData() {
        observeJoinDataUseCase.execute().observe { joinData ->
            when (joinData) {
                is State.Success -> {
                    isSelfUserHardMuted = joinData.data.students.find { it.id == studentUserId }
                        ?.micHardMuted == true
                    if(isSelfUserHardMuted) sharedPreferencesHelper.audioEnabled = false
                    joinSession(joinData.data.toPresentation())
                    if (joinData.data.session?.ended == true) {
                        joinDataFlowSharedFlow.emit(true)
                    }
                }
                State.Loading -> {
                    screenLoadingSharedFlow.emit(State.Loading)
                }
                is State.Error -> {
                    apiErrorSharedFlow.emit(State.Cancelled)
                }
                State.Idle, State.Cancelled, State.StopLoading -> Unit
            }
        }
    }
}

observeJoinDataUseCase is an interface as below

interface ObserveJoinDataUseCase {

    fun execute(): MutableStateFlow<State<DataClass>>

}

Guide me on how to create test case for the View Model and how can i perform test for the subscribeOnJoinData Function