Unknown regression in 3.0.1

[Kame7C0]

Godot version:
3.0.1

OS/device including version:
Windows7 x64

Issue description:
I do not have a clue what is the problem but
provided scene does not run in 3.0.1 and immediately get
** Debug Process Started **
** Debug Process Stopped **
but works fine in 3.0

Steps to reproduce:
Open provided scene "shaders01.tscn" and hit F6

Minimal reproduction project:
1 - testing shaders.zip

[Zireael07]

Have you tried running it several times? I noticed that after upgrading Godot, the first time or two the debugger connection doesn't quite work.

[hpvb]

This is probably a duplicate of #17028. This will be fixed in 3.0.2

[akien-mga]

Indeed, there's no main scene defined so it's a duplicate of #17028.