It is late to answer but for the others;
In my case I assigned a variable at PostInitProperties function. I actually wanted to use PostInitializeComponents function. I assigned a variable of one of the components at that function and that caused this error. After searching a little I see that this function is running before construction of my component. I think I am assigning the value but after component fully created it leaks the assigned value.
I think this is happening at the constructors of objects. You should look at the constructors and see are there any non-assigned unreal engine objects (actors or derived classes etc.). Or are you assigning the object before it is fully created.