Flickable contentY alias in views

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I need to expose vertical scrolling position to the C++ side of my application. If I make a property alias like this

TreeView {
  property alias verticalScrollPosition: flickableItem.contentY
  onDoubleClicked: console.log(flickableItem.contentY)
}

I get the following error: Invalid alias reference. Unable to find id "flickableItem". But if I comment out property declaration and double click any item I get current vertical scrolling printed to a console. What is the reason of this behavior?