Reproduction steps:
1. Open the attached “Unit6Test.zip” project
2. Build and Run for WebGL (File > Build and Run)
3. Observe the Player in the browser and turn on Developer Tools (press F12)

Expected result: “Receivied a message!” Log is visible in the Console
Actual result: “SendMessage: object Main not found!” Log is visible in the Console

Reproducible with: 2023.3.0b6, 6000.0.16f1
Not reproducible with: 2021.3.43f1, 2022.3.43f1, 2023.3.0b5

Reproducible on: Windows 11
Not reproducible on: No other environment tested

Note: The SendMessage works correctly if delay is added at the start