JER adds a FakeClientLevel class that is used as a return value for JERAPI#getLevel(), when Minecraft.getInstance().level is null. However, the FakeClientLevel constructor calls DimensionHelper.getType which in turn calls Minecraft.getInstance().level.registryAccess() throwing a NullPointerException if Minecraft.getInstance().level is null. Therefore FakeClientLevels can only be successfully constructed, if Minecraft.getInstance().level is not null, however in that case JERAPI#getLevel() just returns the existing client.