title: how to map String on Radio
body: <p>I've faced the next issue:</p>
  
  <p>I want to create CRUD-perations in gxt grid, but I've run into a such situation:
  I made <code>sex</code> field as a <code>RadioGroup</code> on the form and wrote a converter for <code>create</code> operation(code below), so it works normally(entity and converter code below):</p>
  
  <pre><code>private Long id;
  private String name;
  private String sex;
  
  formBinding.getBinding(radioGroup).setConverter(new Converter() {
          @Override
          public Object convertFieldValue(Object value) {
              return ((Radio) value).getData("value");
          }
  });
  </code></pre>
  
  <p>Here is code for adding radiogroup to my form:</p>
  
  <pre><code>Radio radio = new Radio();
  radio.setBoxLabel("M");
  radio.setData("value", "M");
  Radio radio2 = new Radio();
  radio2.setBoxLabel("F");
  radio.setData("value", "F");
  
  radioGroup = new RadioGroup();
  radioGroup.setSelectionRequired(true);
  radioGroup.setFieldLabel("Sex");
  radioGroup.add(radio);
  radioGroup.add(radio2);
  radioGroup.setName("sex");
  simplePanel.add(radioGroup, formData);
  </code></pre>
  
  <p>I need to convert the value from String to radio and set it to the <code>radiogroup</code>.
  but when I tried to write the opposite operation, i.e. editing, it throws me such an exception:</p>
  
  <pre><code>11:52:23.381 [ERROR] [hellogxt] Uncaught exception escaped
  java.lang.ClassCastException: java.lang.String cannot be cast to com.extjs.gxt.ui.client.widget.form.Radio
      at com.extjs.gxt.ui.client.widget.form.RadioGroup.setValue(RadioGroup.java:1)
      at com.extjs.gxt.ui.client.binding.FieldBinding.updateField(FieldBinding.java:207)
      at com.extjs.gxt.ui.client.binding.FieldBinding.bind(FieldBinding.java:82)
      at com.extjs.gxt.ui.client.binding.Bindings$1.execute(Bindings.java:92)
      at com.google.gwt.user.client.CommandExecutor.doExecuteCommands(CommandExecutor.java:310)
      at com.google.gwt.user.client.CommandExecutor$2.run(CommandExecutor.java:205)
      at com.google.gwt.user.client.Timer.fire(Timer.java:149)
      at sun.reflect.GeneratedMethodAccessor35.invoke(Unknown Source)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:601)
      at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
      at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
      at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
      at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessagesWhileWaitingForReturn(BrowserChannelServer.java:337)
      at com.google.gwt.dev.shell.BrowserChannelServer.invokeJavascript(BrowserChannelServer.java:218)
      at com.google.gwt.dev.shell.ModuleSpaceOOPHM.doInvoke(ModuleSpaceOOPHM.java:136)
      at com.google.gwt.dev.shell.ModuleSpace.invokeNative(ModuleSpace.java:561)
      at com.google.gwt.dev.shell.ModuleSpace.invokeNativeObject(ModuleSpace.java:269)
      at com.google.gwt.dev.shell.JavaScriptHost.invokeNativeObject(JavaScriptHost.java:91)
      at com.google.gwt.core.client.impl.Impl.apply(Impl.java)
      at com.google.gwt.core.client.impl.Impl.entry0(Impl.java:213)
      at sun.reflect.GeneratedMethodAccessor28.invoke(Unknown Source)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:601)
      at com.google.gwt.dev.shell.MethodAdaptor.invoke(MethodAdaptor.java:103)
      at com.google.gwt.dev.shell.MethodDispatch.invoke(MethodDispatch.java:71)
      at com.google.gwt.dev.shell.OophmSessionHandler.invoke(OophmSessionHandler.java:172)
      at com.google.gwt.dev.shell.BrowserChannelServer.reactToMessages(BrowserChannelServer.java:292)
      at com.google.gwt.dev.shell.BrowserChannelServer.processConnection(BrowserChannelServer.java:546)
      at com.google.gwt.dev.shell.BrowserChannelServer.run(BrowserChannelServer.java:363)
      at java.lang.Thread.run(Thread.java:722)
  </code></pre>
  
  <p>I've tried to add some functionality to my converter, but it seems that binder doesn't invoke my code(formbinder doesn't step into the method at all at edit operation).</p>