I used to have this error on an old project and another version. Try to look for assets in your game using this default material which is Editor only. Try also to exclude every folder which is not supposed to be packaged from packaging in your Packaging settings.
A good way to see if one of your assets you are packaging is referencing this Material, is to right-click on this WorldGridMaterial in the content browser and “Find References”. It should show you every asset referencing this WorldGridMaterial. Do not forget to enable “Show Engine content” and stuff like that in the content browser (can’t remember where this Material is).