Ext.Viewport
is just the class, you need an instance of that class in order to add components to it:
Ext.create('Ext.container.Viewport', {
id: 'myviewport'
});
var paneltab = Ext.create('ROMLProjectAdmin.view.MainTabPanel');
Ext.getCmp('loginregister').destroy();
Ext.getCmp('myviewport').add(paneltab);