bzr reconfigure --standalone breaks externals

Suppose you have a bzr branch 'mybranch' that uses a shared repository 'repo' with externals (that also use the same shared repository) ext1, ext2 etc. in a layout like this:
  /repo
    /mybranch
      /ext1
      /ext2

Now in 'mybranch' you do 'bzr reconfigure --standalone' to stop using the shared repository.

Any operation on the externals will now fail with the error message 'bzr: ERROR: No repository present: "file:///.../repo/mybranch/ext1/"'

Reproducibility: always

Steps to reproduce:
bzr init-repo repo && cd repo
bzr init mybranch && cd mybranch
bzr eadd lp:bzr-externals ext1
# bzr info ext1 will now indicate it uses the shared repository
bzr ci -m "add external"
bzr reconfigure --standalone
# bzr info ext1 will now fail with 'bzr: ERROR: No repository present'

The workaround is to first reconfigure all the externals as standalone branches and only finally reconfigure the branch containing the externals.

A fix would be the externals plugin intercepting reconfigure on the externals' parent branch and first (in a depth-first ordering) reconfigure all the externals.