We faced a problem with the PasswordMask Plugin on a TextField. The TextField is inside a Panel in a TabPanel. If the Tab/Panel with the Password TextField inside gets closed/destroyed the following JS Exception is thrown:
Uncaught TypeError: Cannot read property 'destroy' of null
    at constructor.destroy (<anonymous>:111:26)
    at Object.destroy (ext.axd?v=4.8.1:1557)
    at Object.destroy (ext.axd?v=4.8.1:1555)
    at constructor.doDestroy (ext.axd?v=4.8.1:45778)
    at constructor.callParent (ext.axd?v=4.8.1:8200)
    at constructor.doDestroy (ext.axd?v=4.8.1:117648)
    at constructor.callParent (ext.axd?v=4.8.1:8200)
    at constructor.doDestroy (ext.axd?v=4.8.1:118888)
    at constructor.destroy (ext.axd?v=4.8.1:45723)
    at constructor.callParent (ext.axd?v=4.8.1:8200)
which corresponds to 

    destroy : function () {
        this.callParent(arguments);
        this.hiddenField.destroy();
    },
It looks like the sequence of the function calls is wrong.
When callParent is called this.hiddenField gets destroyed. I think it should be first destroy the hiddenField, then callParent.