Using a loom crashes the game

Can confirm

Code analysis (Yarn mappings)

The EntityModelLayers.STANDING_BANNER model layer does not contain the flag part; instead, this is provided by EntityModelLayers.STANDING_BANNER_FLAG. In previous versions these were all the same EntityModelLayers.BANNER model layer so the issue did not occur.

To fix the issue, the following line in the LoomScreen#init method:

this.bannerField = this.client.getEntityModelLoader().getModelPart(EntityModelLayers.STANDING_BANNER).getChild("flag");

...should be replaced with:

this.bannerField = this.client.getEntityModelLoader().getModelPart(EntityModelLayers.STANDING_BANNER_FLAG).getChild("flag");