fix crashes regarding ExecuteMainThreadActions

[ccp-intern]

No description provided.

[filipppavlov]

It's unfortunate that with this change, we lose the memoizing of those vector allocations.
Looking at the original change to controllers dbe8efd, I see that we lost ScopedBlockTrap that prevented Python/Blue functions from yielding. At the very least, we need to reinstate those, either inside the ExecuteMainThreadActions or around calls to it. And maybe make this function just return immediately if it detects that it's bein reentered?