GitLab

Cleanup the code to prevent this bug from happening again.

Closes #42.

It is now accessible via F6 when on tree view (as a bonus, it is
now also reachable via the mouse). The function bar now dynamically
changes to reflect the toggle nature of the tree-view mode (F5)
and the F6 key serves as expand/collapse when on tree mode,
and its previous behavior of bringing up the "Sort By" menu
(which only made sense on non-tree mode). Users wishing to go to
the "Sort By" menu straight from Tree View can still do so with the
"<" and ">" keys (the top-compatible keys for sort selection).

Add missing check for return value and be more conservative in constructs to avoid future slips of this kind.