When I right-click without a item my game crashes, I'm trying to make a compliment machine that if you right-click on it with cheese it will take 1 cheese and say a compliment in chat, here's my code:

package com.Egietje.degeweldigemod.init;

import java.util.Random;

import javax.annotation.Nullable;

import net.minecraft.block.Block;
import net.minecraft.block.BlockChest;
import net.minecraft.block.material.Material;
import net.minecraft.block.state.IBlockState;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.item.Item;
import net.minecraft.item.ItemStack;
import net.minecraft.stats.StatList;
import net.minecraft.util.EnumFacing;
import net.minecraft.util.EnumHand;
import net.minecraft.util.math.BlockPos;
import net.minecraft.util.text.TextComponentString;
import net.minecraft.world.ILockableContainer;
import net.minecraft.world.World;

public class ComplimentsMachine extends Block {
public Random random = new Random();

public ComplimentsMachine(Material materialIn) {
	super(materialIn);
}

public boolean onBlockActivated(World worldIn, BlockPos pos, IBlockState state, EntityPlayer playerIn, EnumHand hand, @Nullable ItemStack heldItem, EnumFacing side, float hitX, float hitY, float hitZ) {
	int compliment = random.nextInt(2); //chooses a random compliment out of 2(for now)
	Item holdingItem = playerIn.getHeldItemMainhand().getItem(); //detects holding item
	if (worldIn.isRemote) {
		return true;
	} else {
		if (compliment == 0 && holdingItem == CheeseItems.cheese) {
			playerIn.addChatMessage(new TextComponentString("§e" + playerIn.getDisplayNameString() + "§r§6 is AWESOME.§r"));
		} else {
			if (compliment == 1 && holdingItem == CheeseItems.cheese) {
				playerIn.addChatMessage(new TextComponentString("§6I love§r §e" + playerIn.getDisplayNameString() + "§r§6.§r"));
			} else {
				return true;
			}
		}
		return true;
	}
}
}

EDIT: I'm a bit new to coding, but I can understand most things you throw at me

Fixed it by doing:

Item holdingItem = playerIn.getHeldItemMainhand() != null ? playerIn.getHeldItemMainhand().getItem() : null;

Thanks!