PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

If one tries to nest an element inside ActionItem an exception is thrown.
This error is only thrown when action items are nested inside ActionBar.actionItems.
It seems there is a bug in xml2ui module regarding complex properties (ListView.itemTemplate, ActionBar.actionItems, etc).

This will work

<Page.actionBar>
    <ActionBar>
        <!-- Creating custom views for ActionItem-->
        <ActionItem ios.position="right">
                <Button text="Theme"  />
        </ActionItem>
    </ActionBar>
</Page.actionBar>

but this will throw error

<Page.actionBar>
    <ActionBar>
        <ActionBar.actionItems>
            <!-- Creating custom views for ActionItem-->
            <ActionItem ios.position="right">
                    <Button text="Theme"  />
            </ActionItem>
        </ActionBar.actionItems>
    </ActionBar>
</Page.actionBar>

What is the new behavior?

An attempt to fix bug in xml2ui.
This fix allows ActionItems to accept a nested child as described in the docs here: https://v6.docs.nativescript.org/ui/components/action-bar#styling