This is the most common issue you will face as a new user of Specs. This panic will occur whenever a System is first dispatched, and one or more of the components and/or resources it uses is missing from World.

There are a few main reasons for this occurring:

• Forgetting to call setup after building a Dispatcher or ParSeq. Make sure this is always run before the first dispatch.
• Not adding mandatory resources to World. You can usually find these by searching for occurrences of ReadExpect and WriteExpect.
• Manually requesting components/resources from World (not inside a System), where the component/resource is not used by any Systems, which is most common when using the EntityBuilder. This is an artifact of how setup works, it will only add what is found inside the used Systems. If you use other components/resources, you need to manually register/add these to World.