Problems with custom container

[Merthew]

Upon right clicking my custom machine, nothing happens and i get this error:

[11:11:31] [main/FATAL] [net.minecraft.client.Minecraft]: Error executing task
java.util.concurrent.ExecutionException: java.lang.ClassCastException: merthew.mod.objects.machines.furnace.ContainerFurnace cannot be cast to net.minecraft.client.gui.GuiScreen
	at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.8.0_161]
	at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.8.0_161]
	at net.minecraft.util.Util.runTask(Util.java:54) [Util.class:?]
	at net.minecraft.client.Minecraft.runGameLoop(Minecraft.java:1176) [Minecraft.class:?]
	at net.minecraft.client.Minecraft.run(Minecraft.java:441) [Minecraft.class:?]
	at net.minecraft.client.main.Main.main(Main.java:118) [Main.class:?]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_161]
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_161]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_161]
	at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_161]
	at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [launchwrapper-1.12.jar:?]
	at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_161]
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_161]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_161]
	at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_161]
	at net.minecraftforge.gradle.GradleStartCommon.launch(GradleStartCommon.java:97) [start/:?]
	at GradleStart.main(GradleStart.java:25) [start/:?]

Anybody know how to fix this?

[Merthew]

If you need my code:

https://github.com/Merthew/MerthewMod

[V0idWa1k3r]

You must return an object that extends GuiScreen in your GuiHandler#getClientGuiElement. You are returning a Container.