bzr crashes after pulling from branch with subtrees joined by reference (developer-subtree format)

[Florian Rathgeber]

The crash occurs if there are changes in the joined subtree to pull, not otherwise. I'm aware that neither the developer-subtree format nor join --reference are meant for production use atm, but this bug effectively inhibits me using it.

Steps to reproduce (bzr 2.3b3):

mkdir foo && cd foo
bzr init
touch file
bzr add
bzr ci -m "init"
cd ..
mkcd bar && cd bar
bzr init --development-subtree
bzr branch ../foo/
bzr join --reference foo/
bzr ci -m "joined"
cd ..
bzr branch bar/ baz
cd foo
echo "Hello, World" > file
bzr ci -m "modified file"
cd ../bar/foo
bzr pull
cd ..
touch file
bzr add
bzr ci -m "added file"
cd ..
cd baz
bzr pull

The following error occurs:
bzr: failed to report crash using apport:
     OSError(13, 'Permission denied')
bzr: ERROR: exceptions.AttributeError: 'NoneType' object has no attribute 'reference_parent'

The traceback is attached (starting from the final pull)