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.
Problem: When disabling the modules, a warning is thrown and the “Enable” button is still disabled.
A meta data file (.meta) exists but its folder 'Assets/Plugins/MiTschMRStudiosData/Skills' can't be found, and has been created. Empty directories cannot be stored in version control, so it's assumed that the meta data file is for an empty directory in version control. When moving or deleting folders outside of Unity, please ensure that the corresponding .meta file is moved or deleted along with it.
Solution: Locate the folder and delete it.
An empty folder is shown with an empty folder icon.