The issue occurs while running the DW and you arrive at the point where you have a document conflict.
When trying to display that document and the list of possible options to take a velocity error about NullPointerException is shown.
My two cents is that this happens because the script is trying to get a wikiReference from a local object that does not have the wiki parameter set.
Location of problem: displayDocumentReference macro in extension.vm
Stack trace: https://ezcrypt.it/bY8n#pfPbimotWNpmWNaylbluzhCK
Possible solution: https://github.com/vrachieru/xwiki-platform/commit/d5b1ae33aa285d2303aa617316da7c9f6b6c3d50