For full completion functionality - where swcs are included when searching for the completions results - you need to have the 'nokogiri' and 'rubyzip' gems installed. The default ruby shipping on OS X is 1.8.7 so the easiest solution is to switch to the system default ruby and install the nokogiri and rubyzip gems there.
However the following should also work....
TextMate 2, rvm (with ruby 1.9.3 installed) and the AS3 bundle are installed on your machine and when you try to autocomplete an error gets thrown.
Create the gemset the ActionScript 3 bundle needs to be fully functional
rvm gemset create tmas3
rvm use 1.9.3@tmas3
Install the required gems
gem install rubyzip
gem install nokogiri
Create the rvm textmate wrapper
rvm wrapper 1.9.3@tmas3 textmate
Open TextMate preferences and set the var TM_RUBY
to /Users/{YOUR_USER_NAME}/.rvm/bin/textmate_ruby
Restart TextMate and autocompletion should be working, if you suspect that swc's aren't being searched then ~/Library/Logs/TextMate ActionScript 3.log will confirm it.
Actually I'm still having trouble with TextMate 2 and RVM, I think I've narrowed down to the way a project is opened but haven't haven't had the time to write up a bug report. When I open a project using the mate tool on the command line, and TM isn't already open, then completion can still fail...
However your stack trace looks like it's related to another problem - the NilClass should be the value of ENV['TM_CURRENT_LINE'] which is set by TM. I've no idea why it isn't getting set, if you can supply a class/project I can reproduce the problem with then I'll try and take a look.