create method
Creates and returns the T instance. Called once during initialization.
Override this method or supply viewModelFactory or viewModel to the constructor. Throws UnimplementedError if none of the three is used.
Implementation
T create(BuildContext context) {
if (viewModel != null) return viewModel!;
if (viewModelFactory != null) return viewModelFactory!(context);
throw UnimplementedError(
'$runtimeType must either override create(), provide viewModelFactory, '
'or provide viewModel.',
);
}