Problem: You get a NullReferenceException when opening the skill tree and no skills are visible

NullReferenceException: Object reference not set to an instance of an object MiTschMR.Skills.SkillTreeItemsUI.UpdateSkills (...) (at ...)

Solution: Unpack all prefabs in the skill tree by right clicking on the blue gameobjects and selecting "Unpack" --> "Unpack Completely" (may differ depending on your version of Unity). Screenshot shows the skill tree prefab with prefabs in it.