FIX: XML writer document ending

Fixed a bug where the XML writer would end the document with an opening
<posts> tag in place of a closing </posts> tag.
Added stacktrace retrieval method

Added stack trace retrieval method to find posts that contain an exactly
identical stack trace than the one given as input.
Fix: Fixed filtering null pointer issue

Fixed a bug that would make the program crash whenever it tried to read
a non existing attribute in the xml node.

The filter now tries to check if the value it retrieved is not null
before going any further.