[FIX] web: getFocusableElement error in tree view

When a line in a editable tree view has no acivable element
(readonly or toggle_bollean fields only) the currentFieldIndex
will be null after selecting the row, and when trying to edit a
field (boolean field_toggle in this example) an error will be
thrown when trying to put focus on currentWidget.
We need to check that the currentWidget is not undefined.