org.mockito.exceptions.misusing.UnfinishedStubbingException:

This error occurs while running the following code

org.mockito.exceptions.misusing.UnfinishedStubbingException

 private val _forgetPasswordRequest: MutableLiveData<Event<ApiRequest<Unit>>> =
    MutableLiveData()
 private val _confirmPasswordRequest: MutableLiveData<Event<ApiRequest<Unit>>> =
    MutableLiveData()
 private val _changePasswordRequest: MutableLiveData<Event<ApiRequest<Unit>>> =
    MutableLiveData()
 private val _showDialog: MutableLiveData<Event<Unit>> = MutableLiveData()

@Before
fun init() {
    viewModelFactory = Mockito.mock(LoginViewModelFactory::class.java)
    this.fragmentFactory = TestFragmentFactory(this.viewModelFactory!!)
    this.mockNavController = Mockito.mock(NavController::class.java)
    viewModel = Mockito.mock(LoginViewModel::class.java)

    //doReturn(_forgetPasswordRequest).`when`(viewModel!!).forgetPasswordRequest

  /*  `when`(viewModel!!.forgetPasswordRequest).thenReturn(_forgetPasswordRequest)
    `when`(viewModel!!.confirmPasswordRequest).thenReturn(_confirmPasswordRequest)
    `when`(viewModel!!.changePasswordRequest).thenReturn(_changePasswordRequest)
    `when`(viewModel!!.showDialog).thenReturn(_showDialog)*/


    doReturn(_forgetPasswordRequest).`when`(viewModel)?.forgetPasswordRequest
    doReturn(_confirmPasswordRequest).`when`(viewModel)?.confirmPasswordRequest
    doReturn(_changePasswordRequest).`when`(viewModel)?.changePasswordRequest
    doReturn(_showDialog).`when`(viewModel)?.showDialog

    doReturn(viewModelFactory!!.create(saveStateHandler)).`when`(viewModel)

}

What is my mistake?