Thanks for the bug report. This bug is now fixed in the source code repository so it will be fixed in the next release.
There is a temporary workaround that will prevent the error. You can edit the file /Forums/EditPost.aspx
look for this:
<forum:PostList id="postList" runat="server" UseReverseSort="true" />
change UseReverseSort to false, the posts below the editor will be sorted differently but there will be no error.