I setup a Development Environment for this mod, on the same Forge (13.20.0.2216) and pressed the RUN button in Eclipse. It crashed on start, here's the log: http://pastebin.com/KSy2R5zy

Mac OS X 10.11.6 (El Capitan)
Eclipse Neon.2 Release 4.6.2
Forge 1.11.2 (13.20.0.2216)

No, the mcp folder has to be in the folder where your build.gradle file is. The best mapping version would be the one from your workspace yes.

Also, it appears that your item classes are throwing a java.lang.NullPointerException

Caused by: java.lang.NullPointerException
at lumien.randomthings.item.ItemBottleOfAir.(ItemBottleOfAir.java:35)
... 45 more

Actually... java.lang.NoSuchFieldException: field_184128_aI
Field does not exist?

In MCPNames.java, you state that mappingDir is "./../mcp", which is where?

Random-Things/
Random-Things/src/main/
Random-Things/src/main/java
Random-Things/src/main/java/lumien/randomthings

Or?

These errors come from your missing mcp mappings. I already told you, the mcp folder has to be in the same folder where your build.gradle file is. (Or one directory up from your run directory).

It is in the same dir

Random Things needs mcp mappings in a not obfuscated environment. Create a folder called mcp in your "main" project folder and copy fields.csv & methods.csv into it. You get those from here.

It still didn't work.. So I copy fields.csv and methods.csv into the src/main/mcp folder? I also assume I need not the latest version, but the snapshot_20161220, as indicated by your build.gradle?

Could you post the content of the mcp folder?

Huh, that's odd. Could you post your whole fml-client-latest log?

http://pastebin.com/3GuAQHeZ

Github doesn't support .log files

Thanks for your help! I really appreciate that you tried to help me through what I think is a time zone difference.

You also need to add "-Dfml.coreMods.load=lumien.randomthings.asm.LoadingPlugin" to the VM Arguments of your Run Configuration (Run -> Run Configurations)