title: DateTImePickerFormField Does not dismiss keyboard after date selected
body: <p>I am trying to create an age field validator using the DateTimePickerFormField package in flutter. I can make the calendar appear but, I cannot dismiss it after selecting the date. </p>
  
  <p>I have checked on youtube and flutter package site and I'm not sure where I went wrong here</p>
  
  <pre><code>Widget _buildDateField(BuildContext context) {
      final _ageFocus = FocusNode();
    final dateFormat = DateFormat("mm-dd-yyyy");
  
    var _formKey = GlobalKey&lt;FormState&gt;();
    return    Container(
                  width: 120,
                  child: Column(
                    children: &lt;Widget&gt;[
                      Form(
                        key: _formKey,
                        child: Container(
                          child: DateTimePickerFormField(
                            dateOnly: true,
                            format: dateFormat,
                            validator: (val) {
                              if (val != null) {
                                return null;
                              } else {
                                return 'Date Field is Empty';
                              }
                            },
                            decoration: InputDecoration(
                                hintText: 'Age', icon: Icon(Icons.calendar_today)),
                            style: TextStyle(fontWeight: FontWeight.bold),
                            initialDate: DateTime.now(),
                            onSaved: (value) {
                              debugPrint(value.toString());
                              print(value.toString());
                            },
                          ),
                        ),
                      ),
                       RaisedButton(
            onPressed: () {
              if (_formKey.currentState.validate()) {
                _formKey.currentState.save();
              } else {
              }
            },
            child: Text('Submit'),
          )
        ],
      ),
  </code></pre>
  
  <p>I want the user to select the date the comes up and then dismiss the keyboard while leaving the value in the field.</p>