Threading issue

[FeaXR]

I'm currently at part 12-Treading, and I've got a weird issue. I did everything as it's said, my code is exactly like in the provided assets folder, but if I don't lock the modifications Queue in this line:
modifications.Enqueue(Structure.MakeTree(pos, biome.minTreeHeight, biome.maxTreeHeight));
then I get a NullReferenceException when I Dequeue (line 141 in the provided script), and there are nearly no trees in the map, except for a few that are farer away from the player. The Queue that I get is null.
If I lock the Queue, this problem doesn't appear, but then I can't destroy the blocks of the trees.

[FeaXR]

I also tried to swap all my code to the provided ones, but the same error occurs. It worked fine before this part, so it originates definitely somewhere here.

[FeaXR]

Another strange thing: it seems like, that if I lock it, only those trees can't be destroyed, that are on chunk edges.

[VolodyTrekalo-coder]

По моему проблема в подгрузке. Попробуйте Return( указать сектор );

[FeaXR]

Do you mean the return of my MakeTree function? It always returns a non null value