[xtext][ui] Guard against NPE in highlighter

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=453161

Change-Id: I48884651ba8ad3b5d34508ce15cd40a9d495a1d9
[xtend][IDEA] Removed unnecessary caching of a type resolver as an
adapter

Change-Id: I0d11c19e7a9b96c8636df55da1e9de1ba0be1629
Signed-off-by: akosyakov <anton.kosyakov@itemis.de>
Merge "[xtend][IDEA] Removed unnecessary caching of a type resolver as an adapter"