Merging sub-forum may encounter error before its parent forum merged for some modules

Precisely, should be this link: https://community.mybb.com/thread-225560-post-1339544.html#pid1339544

Sorry, just said completely the opposite way.. 👶 One should be able to reproduce it in merging a SMF2 board, by assigning an latter created board (has a bigger id_board) as a child forum of an earlier created forum (has a smaller id_board) in the SMF2 board then running the Merge System. One should be able to reproduce it in merging a SMF2 board, by assigning an earlier created forum (has a smaller id_board) as a child forum of an latter created board (has a bigger id_board) in the SMF2 board then running the Merge System. ~~(Edited to add: looks like SMF2 has an issue of showing sub-forums if the scenario is just as above-mentioned.)~~

I'm working on a fix of the SMF2 module. Other modules should be examined, too. This issue should be mostly relating to the modules that need to convert categories first.

OK, make a checklist (items with no notes don't have a separated category table):

[x] bbpress (no category at all, all forums belong to a category created by the Merge System)
[x] fluxbb (doesn't support sub-forums)
[x] ipb3
[x] ipb4
[x] mybb (~~add a TODO mark where the line IMO is not needed 😆~~ It's not needed but not due to this issue.)
[x] phpbb3
[x] punbb (doesn't support sub-forums)
[x] smf (has fix in the drafted PR)
[x] smf2 (has fix in the drafted PR)
[x] vanilla (no category at all, all forums belong to a category created by the Merge System)
[x] vbulletin3
[x] vbulletin4
[x] vbulletin5
[x] wbb3
[x] wbb4
[x] xenforo
[x] xenforo2

Extra notes:

vbulletin3 (duplicate code can be removed from import_forums module)
vbulletin4 (duplicate code can be removed from import_forums module)
vbulletin5 (duplicate code can be removed from import_forums module)

mybb / merge-system

Merging sub-forum may encounter error before its parent forum merged for some modules #238