[Solved] class cannot access its superclass WWindow

[Guedez 1]

When I run the game through Eclipse, this does not happen

But if I build the mod into a jar for the modloader, this appears

From google, I get that it's something about class loader this or that, but never they explain how to get past this.

Mods like LiveMap and SkillTracker implements WWindow just like I am doing, but that bug does not happen when I load those mods.
What causes this?

 

Edit: Adding "sharedClassLoader=true" to mod properties fix the issue

Edited February 15, 2020 by Guedez