Relase Build Errors

> Task :app:lintVitalRelease
/home/harun/AndroidStudioProjects/droidconKE2020App/features/sessions/src/main/res/layout/fragment_sessions.xml:38: Error: @+id/toolbar2 is not a sibling in the same ConstraintLayout [NotSibling]
                    app:layout_constraintTop_toBottomOf="@+id/toolbar2"
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/harun/AndroidStudioProjects/droidconKE2020App/features/sessions/src/main/res/layout/fragment_sessions.xml:55: Error: @+id/toolbar2 is not a sibling in the same ConstraintLayout [NotSibling]
                    app:layout_constraintTop_toBottomOf="@+id/toolbar2" />
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   Explanation for issues of type "NotSibling":
   Layout constraints in a given ConstraintLayout or RelativeLayout should
   reference other views within the same relative layout (but not itself!)

/home/harun/AndroidStudioProjects/droidconKE2020App/features/sessions/src/main/java/com/android254/droidconKE2020/sessions/ui/views/viewmodel/SessionDetailViewModel.kt:35: Error: Cannot set non-nullable LiveData value to null [NullSafeMutableLiveData]
        _saveSession.value = null
                             ~~~~
/home/harun/AndroidStudioProjects/droidconKE2020App/features/sessions/src/main/java/com/android254/droidconKE2020/sessions/ui/views/viewmodel/SessionDetailViewModel.kt:43: Error: Cannot set non-nullable LiveData value to null [NullSafeMutableLiveData]
        _shareSession.value = null
                              ~~~~
/home/harun/AndroidStudioProjects/droidconKE2020App/features/sessions/src/main/java/com/android254/droidconKE2020/sessions/ui/views/viewmodel/SessionDetailViewModel.kt:51: Error: Cannot set non-nullable LiveData value to null [NullSafeMutableLiveData]
        _clickSpeaker.value = null
                              ~~~~
/home/harun/AndroidStudioProjects/droidconKE2020App/features/sessions/src/main/java/com/android254/droidconKE2020/sessions/ui/views/viewmodel/SessionDetailViewModel.kt:59: Error: Cannot set non-nullable LiveData value to null [NullSafeMutableLiveData]
        _navigateBack.value = null
                              ~~~~

   Explanation for issues of type "NullSafeMutableLiveData":
   This check ensures that LiveData values are not null when explicitly       
            declared as non-nullable.

                   Kotlin interoperability does not support enforcing explicit
   null-safety when using                 generic Java type parameters. Since
   LiveData is a Java class its value can always                 be null even
   when its type is explicitly declared as non-nullable. This can lead        
           to runtime exceptions from reading a null LiveData value that is
   assumed to be                 non-nullable.

6 errors, 0 warnings

> Task :app:lintVitalRelease FAILED

Execution failed for task ':app:lintVitalRelease'.
> Lint found fatal errors while assembling a release target.