Modpack version:
2.3.1

Java version:
1.8.0_251

Description:
I placed down lava with the Thermal Reservoir in the Aether and it placed down a lava source. I placed down a bucket of lava on top of the flowing lava and the game crashed.
When did the crash happen?
When placing down a bucket of lava on top of flowing lava.

Steps to reproduce:

1. Go into the Aether.
2. Place down a lava source with the Thermal Reservoir.
3. Place down a bucket of lava on top of the flowing lava in Survival Mode.
4. The game crashes.

Minecraft crash report:
https://pastebin.pl/view/6d9589c1 (Orginal Crash)
https://pastebin.pl/view/1e43c575 (In a Test World)

How much RAM did you dedicate to your client and how much RAM does your PC have?
I dedicated 8 GBs and my PC has 12 GBs of RAM

I've added a recipe for Aerogel using Tier 1 and 2 Mystical Agriculture Essences, so hopefully fewer people encounter this issue.

Aether is weird with Lava, it tries to not let you place it, but fails, because many modded machines can place it.