add safe assign line to make sure map is not trying to loop on top of undefined

[alansikora]

Fixes issue #142.

[brettle]

This PR will squash the error but will not necessarily produce the correct behavior because the document that is supposed to be changed will not actually be changed.

PR #154 provides a more complete fix to issue #142. It ensures that the document will be changed, only permits the change if the document exists in the current publication context or another one, and also prevents similar problems with removed().

FYI, issue #142 is actually triggered by my brettle:accounts-login-state package.