public class DemoBlock extends BaseEntityBlock {
    [...]
    @Override
    protected InteractionResult useItemOn(ItemStack stack, BlockState state, Level level, BlockPos pos, Player player, InteractionHand hand, BlockHitResult hit) {
        ...
        if (!player.getItemInHand(hand).isEmpty()) {
            ...
        } else {
            // If the player is not holding anything we'll get give him the items in the block entity one by one

            // Find the first slot that has an item and give it to the player
            if (!blockEntity.getItem(1).isEmpty()) {
                // Give the player the stack in the container
                player.getInventory().placeItemBackInInventory(blockEntity.getItem(1));
                // Remove the stack from the container
                blockEntity.removeItem(1);
            } else if (!blockEntity.getItem(0).isEmpty()) {
                player.getInventory().placeItemBackInInventory(blockEntity.getItem(0));
                blockEntity.removeItem(0);
            } else {
                return InteractionResult.TRY_WITH_EMPTY_HAND;
            }
        }
        
        return InteractionResult.SUCCESS;
    }
}