Hello,
I tried to overrid ethe whole GuiWorldSelection with my GUI elements
I created this class but when I call this MC crashes with null pointer ex, at drawing screen.
 

package hu.testmod.gui;

import com.google.common.base.Splitter;
import com.google.common.collect.Lists;
import java.io.IOException;
import javax.annotation.Nullable;

import net.minecraft.client.gui.GuiCreateWorld;
import net.minecraft.client.gui.GuiListWorldSelection;
import net.minecraft.client.gui.GuiListWorldSelectionEntry;
import net.minecraft.client.gui.GuiScreen;
import net.minecraft.client.gui.GuiWorldSelection;
import net.minecraft.client.resources.I18n;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class GuiTestWorldSelection extends GuiWorldSelection
{
    public GuiTestWorldSelection(GuiScreen screenIn) {
		super(screenIn);
	}

	private static final Logger LOGGER = LogManager.getLogger();

    /** The screen to return to when this closes (always Main Menu). */
    protected GuiScreen prevScreen;
    protected String title = "Select world";

    /**
     * Tooltip displayed a world whose version is different from this client's
     */
    private String worldVersTooltip;
    private GuiTestButton deleteButton;
    private GuiTestButton selectButton;
    private GuiTestButton renameButton;
    private GuiTestButton copyButton;
    private GuiListWorldSelection selectionList;

    /**
     * Adds the buttons (and other controls) to the screen in question. Called when the GUI is displayed and when the
     * window resizes, the buttonList is cleared beforehand.
     */
    public void initGui()
    {
        this.title = I18n.format("selectWorld.title");
        this.selectionList = new GuiListWorldSelection(this, this.mc, this.width, this.height, 32, this.height - 64, 36);
        this.postInit();
    }

    /**
     * Handles mouse input.
     */
    public void handleMouseInput() throws IOException
    {
        super.handleMouseInput();
        this.selectionList.handleMouseInput();
    }

    public void postInit()
    {
        this.selectButton = this.addButton(new GuiTestButton(1, this.width / 2 - 154, this.height - 52, 150, 20, I18n.format("selectWorld.select")));
        this.addButton(new GuiTestButton(3, this.width / 2 + 4, this.height - 52, 150, 20, I18n.format("selectWorld.create")));
        this.renameButton = this.addButton(new GuiTestButton(4, this.width / 2 - 154, this.height - 28, 72, 20, I18n.format("selectWorld.edit")));
        this.deleteButton = this.addButton(new GuiTestButton(2, this.width / 2 - 76, this.height - 28, 72, 20, I18n.format("selectWorld.delete")));
        this.copyButton = this.addButton(new GuiTestButton(5, this.width / 2 + 4, this.height - 28, 72, 20, I18n.format("selectWorld.recreate")));
        this.addButton(new GuiTestButton(0, this.width / 2 + 82, this.height - 28, 72, 20, I18n.format("gui.cancel")));
        this.selectButton.enabled = false;
        this.deleteButton.enabled = false;
        this.renameButton.enabled = false;
        this.copyButton.enabled = false;
    }

    /**
     * Called by the controls from the buttonList when activated. (Mouse pressed for buttons)
     */
    protected void actionPerformed(GuiTestButton button) throws IOException
    {
        if (button.enabled)
        {
            GuiListWorldSelectionEntry guilistworldselectionentry = this.selectionList.getSelectedWorld();

            if (button.id == 2)
            {
                if (guilistworldselectionentry != null)
                {
                    guilistworldselectionentry.deleteWorld();
                }
            }
            else if (button.id == 1)
            {
                if (guilistworldselectionentry != null)
                {
                    guilistworldselectionentry.joinWorld();
                }
            }
            else if (button.id == 3)
            {
                this.mc.displayGuiScreen(new GuiCreateWorld(this));
            }
            else if (button.id == 4)
            {
                if (guilistworldselectionentry != null)
                {
                    guilistworldselectionentry.editWorld();
                }
            }
            else if (button.id == 0)
            {
                this.mc.displayGuiScreen(this.prevScreen);
            }
            else if (button.id == 5 && guilistworldselectionentry != null)
            {
                guilistworldselectionentry.recreateWorld();
            }
        }
    }

    /**
     * Draws the screen and all the components in it.
     */
    public void drawScreen(int mouseX, int mouseY, float partialTicks)
    {
        this.worldVersTooltip = null;
        this.selectionList.drawScreen(mouseX, mouseY, partialTicks);
        this.drawCenteredString(this.fontRenderer, this.title, this.width / 2, 20, 16777215);
        super.drawScreen(mouseX, mouseY, partialTicks);

        if (this.worldVersTooltip != null)
        {
            this.drawHoveringText(Lists.newArrayList(Splitter.on("\n").split(this.worldVersTooltip)), mouseX, mouseY);
        }
    }

    /**
     * Called when the mouse is clicked. Args : mouseX, mouseY, clickedButton
     */
    protected void mouseClicked(int mouseX, int mouseY, int mouseButton) throws IOException
    {
        super.mouseClicked(mouseX, mouseY, mouseButton);
        this.selectionList.mouseClicked(mouseX, mouseY, mouseButton);
    }

    /**
     * Called when a mouse button is released.
     */
    protected void mouseReleased(int mouseX, int mouseY, int state)
    {
        super.mouseReleased(mouseX, mouseY, state);
        this.selectionList.mouseReleased(mouseX, mouseY, state);
    }

    /**
     * Called back by selectionList when we call its drawScreen method, from ours.
     */
    public void setVersionTooltip(String p_184861_1_)
    {
        this.worldVersTooltip = p_184861_1_;
    }

    public void selectWorld(@Nullable GuiListWorldSelectionEntry entry)
    {
        boolean flag = entry != null;
        this.selectButton.enabled = flag;
        this.deleteButton.enabled = flag;
        this.renameButton.enabled = flag;
        this.copyButton.enabled = flag;
    }
}

What could be the problem?

47 minutes ago, diesieben07 said:

You extended GuiWorldSelection and copy-pasted all it's code. That makes no sense.

Why? Then how can I override original GuiWorldSelection's buttons without creating new GUI class?