I know that it has something to do with getCapability function in the container class, but I am not sure what it should return for the hoppers to be unable to access it. I've seen a similar topic for 1.12, but how do I make it work with 1.16?

Could you please explain how do I do that? I have this:

@Override
public <T> LazyOptional<T> getCapability(@Nonnull Capability<T> cap, @Nonnull Direction side) {
    if(cap == CapabilityItemHandler.ITEM_HANDLER_CAPABILITY)
    {
        return itemHandler.cast();
    }
    return super.getCapability(cap, side);
}

private IItemHandlerModifiable createHandler()
{
    return new InvWrapper(this);
}

@Override
public void remove() {
    super.remove();
    if(itemHandler != null)
    {
        itemHandler.invalidate();
    }
}

What should I do now? I tried to make the getCapability to return null, but it crashed.

(I'm sorry, I'm still a begginer)

If I return the empty LazyOptional, hoppers still can steal items out of it. The same happens if I don't override it.

Does your tile entity implements ISidedInventory and overrides the associated methods?

I forgot to do it! Thanks for the help everyone! 

No, do not implement ISidedInventory. This is a vanilla system that is replaced by the Forge capabilities system.

21 hours ago, bajtix said:

If I return the empty LazyOptional, hoppers still can steal items out of it. The same happens if I don't override it.

LazyOptional.empty is...empty