Reproduction steps:
1. Open attached project
2. Open the "Main" scene located in "Assets\_Scenes"
3. Create a new C# script and name it
4. Immediately drag and drop the script onto any object in the hierarchy
5. Warning window will appear: "Can't attach script" (watch Repro.gif)

Actual result: Warning window appears: "Can't attach script".

Expected result: Warning windows should not appear.

Note: Issue doesn't reproduce with a new clear project.

Reproduced on: 5.0.4f1, 5.1.4p1, 5.2.4f1, 5.3.0f4, 5.4.0a6