SRadialBox is not radial

Development Programming & Scripting
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1

I am trying to use Slate SRadialBox class. But the shown result in editor is not showing my buttons in radial alignment. The buttons are only on top of each other.

Has somebody succesfully used this class before and has some tipps ?

Here my CPP:

ChildSlot
    [
        SNew(SRadialBox)
            + SRadialBox::Slot()
            [
                SNew(SButton)
                    .OnClicked(this, &SMyWidget::OnMenuItemClicked)
                    [
                        SNew(STextBlock).Text(FText::FromString(TEXT("Option 1")))
                    ]
            ]
            + SRadialBox::Slot()
            [
                SNew(SButton)
                    .OnClicked(this, &SMyWidget::OnMenuItemClicked)
                    [
                        SNew(STextBlock).Text(FText::FromString(TEXT("Option 2")))
                    ]
            ]
	+ SRadialBox::Slot()
	[
		SNew(SButton)
			.OnClicked(this, &SMyWidget::OnMenuItemClicked)
		[
			SNew(STextBlock).Text(FText::FromString(TEXT("Option 3")))
		]
	]              
    ];