When I made my first nether portal by lighting up the obsidian with Flint and Steel under a cave on my single player's save, Minecraft crashed immediately(Minecraft Crash Report).

After that, I tried to open my single player's save but Mincraft kept crashing like this. Then, I found that removing "magnesium-1.1.2.jar" from mods folder is able to let my open the save normally. However, after putting back the "magnesium-1.1.2.jar" to mods folder, I was not able to open the save again.

Therefore, maybe editing the code from magnesium or finding an alternative mod of magnesium can be the solution of my issue.

This has already been fixed but if you're still getting the issue, changing customPortalColor in the inspirations-client.toml file to false will solve it.