line 35:

 

    public WorldType terrainType;

I don't see an = so I will assume no it needs to equal something.

Ok, so I was really stupid.

I shouldn't have copied WorldProvider and also extended it too. --

I found WorldProviderFlat in net.minecraft.world and copied that, changed package declaration and class name, and it works perfectly.

 

As I had portals for that dimension set up, I gave it a go and the dimension changing works too.

 

What a waste of three hours... >

 

The WHOLE custom world provider class is this:

package com.crafter6789.loztwiprincess.world.dimension;

import net.minecraft.world.WorldProvider;

public class WorldProviderTwilightPrincess extends WorldProvider {
    private static final String __OBFID = "CL_00000388";
    public String getDimensionName(){
        return "Hyrule";
    }
}

 

I don't know why I didn't think of this earlier.

 

Edit: reiterating GitHub link

Other mods not updating is a pretty poor excuse to be developing for an old, unsupported version of forge. It just contributes to the cycle of laziness.