This occurs both on new projects, and when upgrading projects from 2018.2 and below to 2018.3

Mac only - does not occur on Windows

Steps to reproduce:
1) Create new project in 18.3 and put spaces in the project name eg "space test"
2) In the new project, create a new C# script
3) Double click to open the script
4) Observe that there are three tabs: "space" "test" and "NewBehaviourScript" (or whatever you named your script) and all three tabs have no content