Enable diff operation; 2 minor bugfixes

* followup Ifa72f031b: re-enables the diff operation
* bugfix in SFUtils::getPageText: do not access null as object
* bugfix in SFFormPrinter::formHTML: increase instance num of multiple instances

Change-Id: I3e41aadd62cc7fcbe0d64bb1c0aefe0fa540db5d
Merge "Enable diff operation; 2 minor bugfixes"