Avoid VerifyError in headless mode

In headless mode, GenericDialog is not a subclass of java.awt.Frame,
and hence it is not a Container, either.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Uncomment the Eyesis Correction plugin

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