A TreeItem control does not have wrap affect same for any Treeitem control in Windows operating system. You have two options and they are the following:
Use tooltip windows to display the long item text by hovering your mouse cursor over the text item.
Resize the Treeitem control to show the long text item.
The latter is not recommended because you may have other controls (e.g. buttons)
that may end up scrolling off the screen.
On systems such as Windows operating systems a Treeitem control does not allow
wrap affect because if it did, it would cause a huge performance hit when control
needs to rearrange all items so that it can display the long text, especially when
the Treeitem control contains thousands of items.
My advice, use tooltip windows or shorten the text.