It looks like one of your streaming levels is somehow a package that contains neither a world nor a redirector to a world.
The assert you’re hitting is inside UWorld::LoadSecondaryLevels and is just doing check(LoadedWorld) which sadly means we can’t tell which package is causing the issue.
// Find the world object in the loaded package.
UWorld* LoadedWorld = UWorld::FindWorldInPackage(LevelPackage);
// If the world was not found, it could be a redirector to a world. If so, follow it to the destination world.
if (!LoadedWorld)
{
LoadedWorld = UWorld::FollowWorldRedirectorInPackage(LevelPackage);
}
check(LoadedWorld);
If you’re using a source build you could change that assert to the following, and it should then report which package is causing an issue:
checkf(LoadedWorld, TEXT("Failed to load streaming world for '%s' from package: %s"), *GetName(), *LevelPackage->GetName());
Honestly though, this should probably be something softer than an assert. Please enter a bug report.