Assertion failed after loading game

Hello.

I have trouble with never-ending crashing when my game ends loading. Log output gives my only one thing about this:

LogWindows:Error: === Critical error: ===
Assertion failed: ClassAddReferencedObjects != NULL [File:D:\Build\++UE4+Release-4.12+Compile\Sync\Engine\Source\Runtime\CoreUObject\Private\UObject\GarbageCollection.cpp] [Line: 1831] 

I’m 100% sure something is wrong with my C++ code so the question is where this assertion can be encountered after loading game (Editor don’t show up yet because of this error).