I can reproduce this when I do not select the subject first. Seems the spellchecker is not initialized then. Reply->Spell->change dictionary->error Reply->select subject->select message text->Spell->change dictionary->ok Initially the subject seems to be never checked unless you select it at least once. The code from TB Bug 1277658 with the null check works around it. Needs spellcheck=ture set for the subject filed so that the editor in the textfield has a spellchecker.
Bug 1069787 Comment 2 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
I can reproduce this when I do not select the subject first. Seems the spellchecker is not initialized then. Reply->Spell->change dictionary->error Reply->select subject->select message text->Spell->change dictionary->ok Initially the subject seems to be never checked unless you select it at least once. The code from TB Bug 1277658 with the null check works around it. Needs spellcheck=true set for the subject filed so that the editor in the textfield has a spellchecker.