When I right click to plant the relevant seed it crashes on a null pointer:

// Hi. I'm Minecraft, and I'm a crashaholic.

Time: 6/21/16 9:15 PM
Description: Unexpected error

java.lang.NullPointerException: Unexpected error
at net.minecraft.item.ItemSeeds.getPlant(ItemSeeds.java:52)
at net.minecraft.block.Block.canSustainPlant(Block.java:1819)
at net.minecraft.item.ItemSeeds.onItemUse(ItemSeeds.java:31)
at net.minecraft.item.ItemStack.onItemUse(ItemStack.java:157)
at net.minecraft.client.multiplayer.PlayerControllerMP.processRightClickBlock(PlayerControllerMP.java:477)
at net.minecraft.client.Minecraft.rightClickMouse(Minecraft.java:1602)
at net.minecraft.client.Minecraft.processKeyBinds(Minecraft.java:2273)
at net.minecraft.client.Minecraft.runTickKeyboard(Minecraft.java:2057)
at net.minecraft.client.Minecraft.runTick(Minecraft.java:1845)
at net.minecraft.client.Minecraft.runGameLoop(Minecraft.java:1119)
at net.minecraft.client.Minecraft.run(Minecraft.java:404)
at net.minecraft.client.main.Main.main(Main.java:118)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.minecraftforge.gradle.GradleStartCommon.launch(GradleStartCommon.java:97)
at GradleStart.main(GradleStart.java:26)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Item class:

public class MysticalCropSeed extends ItemSeeds {

    public MysticalCropSeed(Block crops, Block soil,String regName) {
        super(crops, soil);
        this.setUnlocalizedName(regName);
        this.setCreativeTab(MysticalCrops.cropsTab);
    }
}

Relevant Block class methods:

public class MysticalCropBlock extends BlockCrops implements IGrowable, IPlantable {

    protected Item getSeeds() {
        final Item seeds = CropItems.seedsMap.get(this);

        if(seeds == null) {
            FMLLog.bigWarning("No seeds detected!");
            return new Item();
        }
        return seeds;
    }

    @Override
    public ItemStack getItem(World world, BlockPos pos, IBlockState state) {
        return new ItemStack(getSeeds());
    }

    protected Item getHarvestedItem() {
        final Item harvestedItem = CropItems.harvestedItemMap.get(this);
        if(harvestedItem == null) {
            FMLLog.bigWarning("No drop registered!");
            return new Item();
        }
        return harvestedItem;
    }

    @Override
    public boolean onBlockActivated(World world, BlockPos pos, IBlockState state, EntityPlayer player, EnumHand hand,
                                    ItemStack stack, EnumFacing side, float hitX, float hitY, float hitZ) {
        if(isHarvestReady(state)) {
            if(world.isRemote) {
                return true;
            }

            final ItemStack savedStack = new ItemStack(getHarvestedItem());

            world.setBlockState(pos, state.withProperty(AGE, 0), 7);
            final EntityItem entItem = new EntityItem(world, player.posX, player.posY - 1D, player.posZ, savedStack);
            world.spawnEntityInWorld(entItem);
            entItem.onCollideWithPlayer(player);
            return true;
        }
        return false;
    }
}

The methods to bind the seed to the block:

public class CropItems {
    public static Item[] MCSeeds;
    public static Item[] MCDrops;
    public static HashMap<MysticalCropBlock, Item> seedsMap = new HashMap<MysticalCropBlock, Item>();
    public static HashMap<MysticalCropBlock, Item> harvestedItemMap = new HashMap<MysticalCropBlock, Item>();

    public static Item redstoneCropSeed;

    public static Item regItem(Item item, String regName) {
        item.setRegistryName(regName);
        item.setUnlocalizedName(regName);

        return GameRegistry.register(item);
    }

    public static Item regSeeds(String regName, Block crop) {
        Item item = new MysticalCropSeed(crop, Blocks.FARMLAND, regName);

        seedsMap.put((MysticalCropBlock) crop, item);

        return regItem(item, regName);
    }

    public static Item regDrops(Item item, Block crop) {
        return harvestedItemMap.put((MysticalCropBlock) crop, item);
    }

    public static void loadItemRegistry() {
        redstoneCropSeed = regSeeds("redstoneCropSeed", CropBlocks.redstoneCrop);
        regDrops(Items.REDSTONE, CropBlocks.redstoneCrop);

        MCSeeds = new Item[] {redstoneCropSeed};
        MCDrops = new Item[] {Items.REDSTONE};
    }
}

Never mind. I fixed it by registering both the blocks and items in the preInit of my Common Proxy.