Bug 177497 - it's not possible to type in CSS code embedded in JSP

https://netbeans.org/bugzilla/show_bug.cgi?id=177497

Description: User cannot type text into css section in embedded html 
code, for example in JSPs

Cause: a bug in lexer #177501 (Cannot get embedded tokensequence from
embedded tokensequence, at least under some circumstances) + a missing 
null check in css editor

Fix: added a simple null check into the css editor, the lexer issue will 
not be addressed in 6.8

Risk: safe

Code reviewed and approved by QE already

http://hg.netbeans.org/web-main/rev/3f51dd9f9384

Thanks,
Marek