Add topic selection to inline thread editing, fixing errors

Co-Authored-By: Brian Talbot <btalbot@edx.org>

TNL-549
Merge pull request #5577 from edx/jsa/tnl549

Fix topic-related errors when editing discussion threads in inline mode.
Fixed errors in discussion XModule, caused by c2716d4 [TNL-606]

Co-Authored-By: jmclaus <jmc@edx.org>
Co-Authored-By: jsa <jsa@edx.org>