Add ChildActorComponent crashing engine

Hello
this is the crash error:

82 ***** 	UE_CLOG(!CurrentInitializer, LogObj, Fatal, TEXT("No object initializer found during construction."));
...
<CALLSTACK START>
UE4Editor_Core!FDebug::AssertFailed() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\core\private\misc\outputdevice.cpp:374]
UE4Editor_CoreUObject!UObject::CreateDefaultSubobject() [d:\buildfarm\buildmachine_++depot+ue4-releases+4.10\engine\source\runtime\coreuobject\private\uobject\obj.cpp:83]

this is what causes the crash (second line to be precise):

UChildActorComponent * ChildActor = CreateDefaultSubobject<UChildActorComponent>(TEXT("test"));
ChildActor->SetChildActorClass(testClass);

This is how I find and pass the actor blueprint to the c++ actor that should spawn it.
//cpp constructor

static ConstructorHelpers::FClassFinder<AActor> testClassFinder(TEXT("/Game/Blueprints/testBP"));
if (testClassFinder.Class != nullptr)
	testClass = testClassFinder.Class;

//header

TSubclassOf<AActor> testClass;

Does anyone have an idea?
Thanks