title: Rxdart BehaviorSubject make its value null
body: <p>I have a wired problem with Rxdart <code>BehaviorSubject</code>! I created a screen in flutter that has two <code>TextField</code> and one <code>DropdownButton</code> and I receiving their data with bloc pattern. </p>
  
  <p>But when I change <code>TextField</code>s values, The value of <code>BehaviorSubject</code> that connected to <code>DropdownButton</code> get null and when I change <code>DropdownButton</code> value, the value of two <code>BehaviorSubject</code>s that connected to <code>TextField</code>s gets null.</p>
  
  <p>Also it does not affect widget rendering. </p>
  
  <p>Can anyone help me understand what am I doing wrong here. here is my bloc code:</p>
  
  <pre><code>class Bloc {
    final _type = BehaviorSubject&lt;String&gt;();
    final _amount = BehaviorSubject&lt;String&gt;();
    final _title = BehaviorSubject&lt;String&gt;();
  
    Stream&lt;String&gt; get type =&gt; _type.stream;
    Stream&lt;String&gt; get amount =&gt; _amount.stream;
    Stream&lt;String&gt; get title =&gt; _title.stream;
  
    Function(String) get changeType =&gt; _type.sink.add;
    Function(String) get changeAmount =&gt; _amount.sink.add;
    Function(String) get changeTitle =&gt; _title.sink.add;
  
    createTransaction() {
      final validType = _type.value;
      final validAmount = _amount.value;
      final validTitle = _title.value;
  
      print(validType);
      print(validAmount);
      print(validTitle);
    }
  
    dispose() {
      _type.close();
      _amount.close();
      _title.close();
    }
  
  }
  </code></pre>