just wanna confirm that you tried deleting the old doodle studio folder and downloading the latest package. the double assets path is definitely the problem and something that was directly fixed in the latest update, let me know!
I deleted the whole DoodleStudio folder, installed the new version and reimported my whole project twice. I haven't tried testing it in a new empty project, though. Not sure if anything can be left over in assets or library somehow... I can try a completely new project and install later today.
I just made a completely new project in Unity 2018.3.6 and imported the plugin and I still get the same error.
I also see this slight smearing across some of the buttons just below the text. Doesn't seem to impact any of the actual doodles I've made so perhaps it's just how it renders UI in the editor: