I have custompage.ui file(simple text) and I am adding it in my package.xml:

<UserInterfaces>
        <UserInterface>custompage.ui</UserInterface>
</UserInterfaces>

And this is how I am using it in my componentscript.js:

Component.prototype.componentLoaded = function ()
{
    installer.addWizardPage(component, "CustomPage", QInstaller.ReadyForInstallation)
}

The problem is that the page is displayed only when I install the application. When I uninstall it, CustomPage is not displayed.

Also, with another approach, if I try to add the customized page in my controlscript.js like this:

Controller.prototype.ReadyForInstallationPageCallback = function ()
{
    try {
        installer.addWizardPage(component, "CustomPage", QInstaller.ReadyForInstallation);
    }
    catch (e) {
        QMessageBox.warning("QMessageBox", "", e, QMessageBox.Ok);
    }
}

I am getting this error:

ReferenceError: component is not defined

So, it looks like the component is not loaded at all when the application is uninstalled.
And from Qt documentation we can add custom pages only in components xml file with <UserInterfaces> tag.
Does this mean that we can not use custom gui pages in the uninstaller or I am missing something?