Navigation in the configuration UI's file tree is not working using arrow keys

[mariusoe]

The navigation in the file tree of the configuration UI is not working using the arrow keys. If a user selects an element, e.g. a file, it is expected to select the next file when pressing the "down-arrow"-key or expanding or collapsing an directory using the left and right key, respectivelly.

Currently, the UI throws an execption when a user presses one of the arrow keys, if an element of the file tree is selected.