Run runClient error

[XDKara]

Hello, when I run runClient (or server) it shows this output. It was on first start (never  started without problems before).

I was reinstalling JDK and googling it but I didn't find anything. I didn't find anybody with this errror.

[2021-01-25 18:47:55] [INFO   ] Natives: C:\Users\Jakub\Desktop\testing\build\natives 
[2021-01-25 18:47:55] [INFO   ] Main Class: net.minecraft.launchwrapper.Launch 
[2021-01-25 18:47:55] [INFO   ] Srg2Mcp: C:\Users\Jakub\Desktop\testing\build\createSrgToMcp\output.srg 
[2021-01-25 18:47:55] [INFO   ] Extra: [] 
[2021-01-25 18:47:55] [INFO   ] Running with arguments: [--version, 1.12, --assetIndex, 1.12, --assetsDir, C:\Users\Jakub\.gradle\caches\forge_gradle\assets, --accessToken, {REDACTED}, --userProperties, [], --tweakClass, net.minecraftforge.fml.common.launcher.FMLTweaker] 
Exception in thread "main" java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:564)
	at net.minecraftforge.legacydev.Main.start(Main.java:86)
	at net.minecraftforge.legacydev.MainClient.main(MainClient.java:29)
Caused by: java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')
	at net.minecraft.launchwrapper.Launch.<init>(Launch.java:34)
	at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
	... 6 more

have I attach some files?

I'm relatively new to programming (I was programming some basic MC plugins), so I will be grateful for any help.