Is there any way to check whether the item a player is holding is a block? I tried

instanceof Block

but it said I couldn't convert Items into Blocks.

Items and Blocks aren't the same thing.

 

Example code

public void onPlayerTick(TickEvent.PlayerTick event)
{
    if(event.side.isServer() && event.phase == TickEvent.Phase.START)
    {
        ItemStack stack = event.player.getCurrentEquippedItem();
        if(stack != null && stack.getItem() != null && Block.getBlockFromItem(stack.getItem()) != null)
        {
            Block block = Block.getBlockFromItem(stack.getItem());
        }
    }
}

Items are the ones in your inventory.

Blocks are the ones in the world.

 

So there is never actually a Block in your inventory, but the Item version of it. You need to use

if(theItem instanceof ItemBlock)

The ItemBlock thing makes sense, but it is still giving me a NullPointerException.

You could probably use something like this:

if(player.getCurrentEquippedItem() != null && player.getCurrentEquippedItem().getItem() instanceof ItemBlock){//DO STUFF}