Deselection in tables and trees is a feature we're still missing; somehow, it has never been a critical issue.
Nonetheless, we do plan to support this in the near future.
In the meantime, you can model a less-than-perfect alternative, as follows:
When this process is executed, you'll notice that the selected row loses its 'selected' style, but note also that <Selected Row> will now contain an empty row, whereas the expected behavior when no row is selected should be that it contains nothing (the 2 situations can be distinguished by using an Exists template which would exit through <Yes> in the first case, and <No> in the second).
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