[RN] Fix assigning Dialog state

react/features/base/dialog/components/Dialog.native.js

  * Implements {@code AbstractDialog} on react-native using {@code Prompt}.
  */
 class Dialog extends AbstractDialog<Props, State> {
+    state = {
+        text: ''
+    };
+
     /**
      * Initailizes a new {@code Dialog} instance.
      *
     constructor(props: Object) {
         super(props);
 
-        this.state = {
-            text: ''
-        };
-
         // Bind event handlers so they are only bound once per instance.
         this._onChangeText = this._onChangeText.bind(this);
         this._onSubmit = this._onSubmit.bind(this);