package mod.flatcoloredblocks.gui;

import net.minecraft.client.gui.GuiScreen;
import net.minecraftforge.fml.client.FMLConfigGuiFactory;

/**
 * Basic Config Factory.
 */
public class ConfigGuiFactory extends FMLConfigGuiFactory
{

	@Override
	public Class<? extends GuiScreen> mainConfigGuiClass()
	{
		return ConfigGui.class;
	}

}

package mod.flatcoloredblocks.gui;

import net.minecraft.client.gui.GuiScreen;
import net.minecraftforge.fml.client.FMLConfigGuiFactory;

/**
 * Basic Config Factory.
 */
public class ConfigGuiFactory extends FMLConfigGuiFactory
{

	@Override
	public GuiScreen createConfigGui(
			GuiScreen parentScreen )
	{
		return new ConfigGui( parentScreen );
	}

}

Use createConfigGui instead of mainConfigGuiClass
Use createConfigGui instead of mainConfigGuiClass
