Closing a tab tree swiitches focus to its no-longer-existent child

Bug Report

Steps to reproduce:

1. Switch to vertical mode.
2. Open at least 3 different tabs in one window.
3. Drag one of the tabs onto another one in order to create a tree.
4. Collapse the tree.
5. Switch to the parent tab of the tree so that it becomes active.
6. Close the parent tab using any of the available methods (Cmd+W, clicking the "X" button, middle-clicking the tab).

You will see that the child of the parent tab has become active, but because it no longer exists, Orion is now displaying an empty page.

Expected behavior:
The focus should go to the tab that would normally receive it if the tab being closed didn't have any children.

Orion/macOS:
0.99.107.7-beta

Image/Video: