Hit javascript error when select more than 2 numberic list & click on insert horizontal line

[jsflim]

Step to reproduce:

1. Select more than 2 lines with inserted numberic list format
2. click on insert horizontal line
3. Hit javascript error

[tobiasz.cudnik]

Confirming on FF 3.6, stacktrace below.

append()
appendTo()
fixBlock()
splitBlock()
onInsertElement
listenerFirer()
(?)()
fire()
insertElement()
exec()
exec()
execCommand()
(?)()
execute()
addFunction()
callFunction()
onclick