450727: NPE due to ThemeDefinitionElement violates supertype's API contract

Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=450727

Change-Id: I4861cd1eaa188834743364c2df6fd7c133fc3e6b
Signed-off-by: Timo Kinnunen <timo.kinnunen@gmail.com>