TopicRoomMessage and ArchiveRoomMessage message type need to be skipped otherwise you get the undefined method error during the import, topics and archived state are not handle by mattermost bulk import.
/usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/hipchat/post.rb:7:in `initialize': undefined method `[]' for nil:NilClass (NoMethodError)
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/hipchat/post_repository.rb:32:in `new'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/hipchat/post_repository.rb:32:in `block in load'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/hipchat/post_repository.rb:27:in `each'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/hipchat/post_repository.rb:27:in `load'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/cmds/room.rb:76:in `tap'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/cmds/room.rb:76:in `block (2 levels) in save'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/cmds/room.rb:70:in `each'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/cmds/room.rb:70:in `block in save'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/cmds/room.rb:50:in `open'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/cmds/room.rb:50:in `save'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/cmds/room.rb:43:in `import'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost/cmds/room.rb:19:in `run'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/lib/hipmost.rb:12:in `run'
from /usr/lib64/ruby/gems/2.1.0/gems/hipmost-0.1.0/exe/hipmost:48:in `<top (required)>'
from /usr/bin/hipmost:23:in `load'
from /usr/bin/hipmost:23:in `<main>'
TopicRoomMessage and ArchiveRoomMessage message type need to be skipped otherwise you get the undefined method error during the import, topics and archived state are not handle by mattermost bulk import.