Defensive programming in DefaultLegacyService

When run in ij-legacy's tests, we cannot assume that there is a default
user interface at all.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
wip
WIP: Verify that Switch to Modern Mode works

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>