Show on App Launch: Fix option observation race condition (#5307)

Task/Issue URL:
https://app.asana.com/0/488551667048375/1208815135507669/f

### Description

Added a null check to the ViewState when attempting to update via
observing the selected ShowOnAppLaunch option.

### Steps to test this PR

- [ ] Launch the app and navigate to General Settings
- [ ] There should be no crash
- [ ] Open the Show on App Launch screen
- [ ] Change your option
- [ ] Navigate back
- [ ] The new Show on App Launch function should still be reflected on
the General Settings screen

### UI changes
No UI changes

app/src/main/java/com/duckduckgo/app/generalsettings/GeneralSettingsViewModel.kt

@@ -154,7 +154,7 @@ class GeneralSettingsViewModel @Inject constructor(
     private fun observeShowOnAppLaunchOption() {
         showOnAppLaunchOptionDataStore.optionFlow
             .onEach { showOnAppLaunchOption ->
-                _viewState.update { it!!.copy(showOnAppLaunchSelectedOption = showOnAppLaunchOption) }
+                _viewState.value?.let { state -> _viewState.update { state.copy(showOnAppLaunchSelectedOption = showOnAppLaunchOption) } }
             }.launchIn(viewModelScope)
     }