show a value dialog

def call(self):
        '''show a value dialog'''
        from MAVProxy.modules.lib.wx_loader import wx
        try:
            dlg = wx.TextEntryDialog(None, self.title, self.title, defaultValue=str(self.default))
        except TypeError:
            dlg = wx.TextEntryDialog(None, self.title, self.title, value=str(self.default))
        if dlg.ShowModal() != wx.ID_OK:
            return None
        return dlg.GetValue()