Unexpected NullPointerException when using ViewModel in Android Studio 4.2.2 with LiveData
I'm facing a `NullPointerException` when trying to observe a `LiveData` object in my Activity. I'm using Android Studio 4.2.2 with Kotlin and the following dependencies in my `build.gradle`: ```groovy implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.3.1" implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.3.1" ``` In my `ViewModel`, I have a `MutableLiveData` that I'm trying to expose: ```kotlin class MyViewModel : ViewModel() { private val _data = MutableLiveData<String>() val data: LiveData<String> get() = _data fun fetchData() { _data.value = "Hello, World!" } } ``` In my `MainActivity`, I'm attempting to observe this `LiveData` in the `onCreate` method: ```kotlin class MainActivity : AppCompatActivity() { private lateinit var viewModel: MyViewModel override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) viewModel = ViewModelProvider(this).get(MyViewModel::class.java) viewModel.data.observe(this, Observer { value -> // This line throws a NullPointerException println(value.length) }) viewModel.fetchData() } } ``` When I run the app, I get the following error: ``` java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference ``` It seems like the LiveData starts out as null, but I'm calling `fetchData()` right after setting the observer. I've confirmed that `fetchData()` is executed, yet the observer still receives a null value. I've also tried using `viewModel.data.value?.let { ... }` to guard against null, but I would like to understand why this is happening in the first place. Is there a best practice to ensure that observers don't encounter null values upon creation? Any advice or insights would be greatly appreciated. This is for a service running on Ubuntu 20.04. Could this be a known issue? This is part of a larger application I'm building. Any feedback is welcome!