Fix crash returning to a live story with unread comments

The stories reply area deletes the comments toggle button when it moves
to a story that isn't live. The new comments dot is a child of that
button, so it was destroyed too, but ComposeControls kept the pointer
to it. Back on a live story, the new button saw that pointer and didn't
create a dot, and the next comments state change deleted the destroyed
dot again. This crashed as soon as the comments of the rejoined stream
loaded.

Clear the dot pointer and delete the previous button before creating a
new one. Moving from one live story straight to another no longer
leaves the old button and its subscriptions behind.