title: Rx<User>() missing argument - Flutter / GetX
body: <p>While using a GetxController on a class, and using Rx to get a FirebaseUser, I'm being shown an error, that I'm missing a positional argument</p>
  <pre class="lang-dart prettyprint-override"><code>class AuthController extends GetxController {
    FirebaseAuth _auth = FirebaseAuth.instance;
    Rx&lt;User&gt; _firebaseUser = Rx&lt;User&gt;();
    User get user =&gt; _firebaseUser.value;
  </code></pre>
  <p>Here on this <strong>Rx&lt;User&gt;()</strong> which is from package:firebase_auth/firebase_auth.dart, I get the error :</p>
  <p><em>1 positional argument(s) expected, but 0 found.
  Try adding the missing arguments.dartnot_enough_positional_arguments</em></p>
  <p>After launching the emulator, I get this more detailed error :</p>
  <p><em>lib/core/controllers/auth_controller.dart:9:36: Error: Too few positional arguments: 1 required, 0 given.
  Rx _firebaseUser = Rx();</em></p>
  <p><em>../../../snap/flutter/common/flutter/.pub-cache/hosted/pub.dartlang.org/get-4.1.3/lib/get_rx/src/rx_types/rx_core/rx_impl.dart:324:3: Context: Found this candidate, but the arguments don't match.
  Rx(T initial) : super(initial);</em></p>
  <p>I can't find what I'm supposed to add here, and every tutorial I saw, didn't specify any argument here.
  Anyone knows the problem ?</p>