Why does not show checkbox in Extjs 6?

I'm developing an application with Extjs-6. I define a checkbox as follow:

{
   xtype: 'checkboxfield',
   fieldLabel: 'This is a CHECKBOX, but it\'s  not display',
   style: {
      width: '95%',
      marginBottom: '10px',
      color: 'blue'
   },
}

But it show as follow:

It display the fieldLabel, but don't show the checkbox. When I define change listeners, and then I press tab key, and then press spacekey, change listeners fires.

Where is the problem?
I set styles, But it change nothing. What do I do?