Checkbox fails if its hide() method is called whilst not rendered

[fabriciomurta]

Found: 4.7.1 Ext.NET forums thread:

The overrides for issues #1550 and #1567 lacks a check for me.rendered at run time, and this causes the methods to trigger exceptions when called to non-rendered instances of checkbox components.

So just adding the test should do it.

As this is coming from Ext.NET overrides, this issue is not coming from Sencha.