If minecraft crashes quickly the game output will be empty

The bug

If Minecraft crashes quickly after starting, the game output will be empty. 

Steps to reproduce

1. Add -javaagent:missing to the JVM arguments
2. Launch an already used installation of Minecraft

Expected result

Minecraft fails to launch and the game output contains something like

22:25:28.024Using default game log configuration client-1.12.xml (outputs XML)22:25:28.260Error: Could not find or load main class "-javaagent:missing"
22:25:28.261Process crashed with exit code 1

Actual result:

Minecraft fails to launch, but game output only shows "Waiting for log output..."

Workarounds

If you cause something to be download (for instance, by deleting the relevant folder in .minecraft/versions) then the log will show the output.

The launcher_log.txt file will still contain the game output.