Sometimes, when I open my customGui, the server shuts down and shows this error:

 

Stacktrace:
at net.minecraft.client.gui.GuiScreen.drawWorldBackground(GuiScreen.java:396)
at net.minecraft.client.gui.GuiScreen.drawDefaultBackground(GuiScreen.java:391)
at com.guje.test_mod.TestBlockGui.drawScreen(TestBlockGui.java:71)

"com.guje.test_mod.TestBlockGui.drawScreen(TestBlockGui.java:71)"

 

Well that's the line your error is on, and something about it is null, feel like providing code so we can help you?

In line 71 there is only

 

drawDefaultBackground();

 

rest of the class:

package com.guje.test_mod;

import org.lwjgl.opengl.GL11;

import net.minecraft.block.Block;
import net.minecraft.client.gui.GuiButton;
import net.minecraft.client.gui.GuiScreen;
import net.minecraft.client.gui.GuiTextField;
import net.minecraft.util.ResourceLocation;
import net.minecraft.world.World;

public class TestBlockGui extends GuiScreen {

private int textureX;
private int textureY;
private int posY;
private int posX;
private int bY;
private int bX;
private int bZ;
private ResourceLocation texture;
private World world;
private String blockId;
private String blockMetadata;
private GuiTextField text_BlockId;
private GuiTextField text_BlockMetadata;

public TestBlockGui(Block block, World world, int bX, int bY, int bZ) {
	super();
	this.textureX = 175;
	this.textureY = 221;
	this.bX = bX;
	this.bY = bY;
	this.bZ = bZ;
	this.world = world;
	this.blockId = block.getIdFromBlock(block)+"";
	this.blockMetadata = world.getBlockMetadata(bX, bY, bZ)+"";
	texture = new ResourceLocation(Hack_n_Mine.MODID, "textures/gui/hackingGui.png");
}

@Override
public void initGui() {
	buttonList.clear();
	posX = (this.width - textureX) / 2;
	posY = (this.height - textureY) / 2;
	text_BlockId = new GuiTextField(fontRendererObj, posX + 77, posY + 5, 40, 10);
	text_BlockId.setText(blockId);
	text_BlockId.setMaxStringLength(5);
	text_BlockMetadata = new GuiTextField(fontRendererObj, posX + 77, posY + 18, 40, 10);
	text_BlockMetadata.setText(blockMetadata);
	text_BlockMetadata.setMaxStringLength(5);
	super.initGui();
}

@Override
protected void keyTyped(char c, int i) {
	text_BlockId.textboxKeyTyped(c, i);
	text_BlockMetadata.textboxKeyTyped(c, i);
	super.keyTyped(c, i);
}

@Override
protected void mouseClicked(int i, int j, int f) {
	text_BlockId.mouseClicked(i, j, f);
	text_BlockMetadata.mouseClicked(i, j, f);
	super.mouseClicked(i, j, f);
}

@Override
public void drawScreen(int x, int y, float f) {
	drawDefaultBackground();
	GL11.glColor4f(1F,1F,1F,1F);
	posX = (this.width - textureX) / 2;
	posY = (this.height - textureY) / 2;

	mc.renderEngine.bindTexture(texture);
	drawTexturedModalRect(posX, posY, 0, 0, textureX, textureY);
	text_BlockId.drawTextBox();
	text_BlockMetadata.drawTextBox();
	fontRendererObj.drawString("BlockID", posX + 5, posY + 7, 0x404040);
	fontRendererObj.drawString("BlockMetadata", posX + 5, posY + 19, 0x404040);
	super.drawScreen(x, y, f);
}

@Override
public void onGuiClosed() {
	//Set blockId and metadata
	Block b = Block.getBlockById(Integer.parseInt(text_BlockId.getText()));
	int m = Integer.parseInt(text_BlockMetadata.getText());
	world.setBlock(bX, bY, bZ, b, m, 3);
	super.onGuiClosed();
}

}

in this function

 

    @Override
    public boolean onItemUse(ItemStack item, EntityPlayer player,
    		World world, int x, int y, int z,
    		int par7, float xFloat, float yFloat,
    		float zFloat) {
    	if (!player.canPlayerEdit(x, y, z, par7, item)){
    		return false;
    	}
    	if (!world.isRemote){
    		Block target = world.getBlock(x, y, z);
    		Minecraft.getMinecraft().displayGuiScreen(new TestBlockGui(target,world,x,y,z));
    	}
    	return true;

    	if (!world.isRemote){
    		Block target = world.getBlock(x, y, z);
    		Minecraft.getMinecraft().displayGuiScreen(new TestBlockGui(target,world,x,y,z));
    	}

 

You're checking to see if it's on the server side, isRemote returns true for client and false for servers. Minecraft is a client only class.

So how can I fix this?

Really? Change !world.isRemote to world.isRemote

I thought about that. But when i change it like that, there aren't any changes made to the server, so it doesn't work

If you're opening a GUI like that you have to do it on the client side, if your GUI is going to interact with the server, you will probably need to use packets to tell the server what to do.

Thanks, i will try that!