SlaveAPI seems to have half-assed bug 1023997, then maybe it failed filing the "is inaccessible" bug and got confused, and now when that slave is rebooted, the result is "'NoneType' object has no attribute 'id_'".
Doubling down on that bet, because of bug 1024169 and the lack of any unreachable bug or second comment there either.
Jordan was looking into trying to fix this.
this bug should be resolved. slaveapi has now been updated to 1.1.2 to reflect the fix. Will leave open for a day of testing. more details can be found https://bugzilla.mozilla.org/show_bug.cgi?id=932396#c30