When I was playing in a 19w06a minecraft world, the game suddenly shut down, and thrown out an exception which look like the following:
---- Minecraft Crash Report ----
// Quite honestly, I wouldn't worry myself about that.
 
Time: 2/13/19 12:23 PM
Description: Ticking entity
.........
– Entity being ticked –
Details:
Entity Type: minecraft:trader_llama (anw)
Entity ID: 256
Entity Name: Trader Llama
Entity's Exact location: -512.47, 75.00, 754.50
Entity's Block location: World: (-513,75,754), Chunk: (at 15,4,2 in -33,47; contains blocks -528,0,752 to -513,255,767), Region: (-2,1; contains chunks -64,32 to -33,63, blocks -1024,0,512 to -513,255,1023)
Entity's Momentum: 0.00, -0.08, 0.00
Entity's Passengers: []
Entity's Vehicle: ~_ERROR~ NullPointerException: null
 
So it seems like that minecraft doesn't like a trader llama to spawn with vehicle:null, however it is how trader llamas work. Seem randomly happen(not always).