[ Please replace text in [] with the answer from the above question! ]

Do you use Java 8?
[Java 21]

What version of Minecraft do you use?
[1.12.2]

What version of MinecraftForge do you use?
[Cleanroom 0.3.2]

What version of Immersive Engineering do you use?
[0.12-98]

What version of Immersive Cables do you use?
[1.3.2]

What is the issue / bug you have encountered? Give us a short description.
[Crash on startup]

How can it be reproduced?
[Install AE2 and prevent materials in its config file

Please post your log file (either in /logs/fml-client-latest.log or /logs/latest.log) on https://gist.github.com and provide the link:
[Crash Report]

Extra info I'll add :
Crashes only when AE2 materials are not loaded, method causing the crash is :

java.util.Objects.requireNonNull(Objects.java:233)

To fix the crash, I removed the Recipes folder in the jar file (editing as if it was a zip) (I'll add them with Crafttweaker).
In the mod itself, I would recommend removing the method causing the crash as ghost items are easier to deal with for pack makers than crashes. Alternatively, a config option to remove default crafting recipes (as they crash the game when the items aren't loaded would exable pack makers to solve this issue.