SidedInvWrapper is a capability wrapper for WorldlyContainer, a vanilla interface with the following definition:

public interface WorldlyContainer extends Container {
    int[] getSlotsForFace(Direction side);

    boolean canPlaceItemThroughFace(int index, ItemStack itemStack, @Nullable Direction direction);

    boolean canTakeItemThroughFace(int index, ItemStack stack, Direction direction);
}

As a capability wrapper for the IItemHandler capability, SidedInvWrapper directly accepts a null Direction and passes it through to these methods, which is illegal in two of three cases.

These erroneous calls are in SidedInvWrapper.getSlot and SidedInvWrapper#extractItem. In both cases, SidedInvWrapper needs to fallback to the equivalent behavior from InvWrapper when the direction is null.

This behavior has gone unnoticed for quite a while, simply because none of the vanilla implementation crash when passed null values. This does not change the fact that this is a bug in SidedInvWrapper.