Update: Loading the mod through eclipse crashes, but loading ./gradlew runClient in PowerShell does not. This might be caused by some problem with the way the runClient.launch file is created.
PowerShell, for whatever reason, does not run into the error. I checked using gradlew runClient in the regular Windows Command Prompt and it also does not have the error.
However, when I load in PowerShell, the game doesn't recognize any of the items and tags in the recipes, as if none of them are added.