Please review fix for issue #193729

#193729 - NullPointerException at 
org.netbeans.modules.refactoring.php.findusages.WhereUsedElement.create
http://netbeans.org/bugzilla/show_bug.cgi?id=193729

The NPE is cased by the fact that the document for a data object is not 
opened correctly and then the content of the document is null. This can 
happen when the file is in different encoding that is the project 
encoding and is not possible read the file in project encoding.

User during a refactoring action see error diallog  with NPE in such 
situation.

The fix corrects the NPE and is simple. Also I added logger to log such 
situation.

Changesets:
Fix:http://hg.netbeans.org/web-main/rev/dcd173f0ae37
Logging:http://hg.netbeans.org/web-main/rev/6948d24b1a4f

Regards,
Petr