Ticking entity − Entity's vehicle: NullPointerException

With this current snapshot, several crashes happened while ticking Entity's vehicle of sheeps referencing a null object.
All the concerned sheeps were not even supposed to be there, they appear at regular intervals on a specific path that has been taken before to transport leashed sheeps. So this might be related to entity tracking.

A detailled walkthrough of this issue is below. No game modification.

java.lang.NullPointerException: Ticking entity

-- Head --
Thread: Server thread
Stacktrace:
        at aho.a(SourceFile:1008)
        at aho.dq(SourceFile:997)
        at ahu.dq(SourceFile:70)
        at aho.h(SourceFile:287)
        at bcq.a(SourceFile:963)
        at ur.a(SourceFile:365)
        at bcq.f(SourceFile:943)

-- Entity being ticked --
Details:
        Entity Type: minecraft:sheep (anb)
        Entity ID: 134409
        Entity Name: Sheep
        Entity's Exact location: 558.98, 69.00, -215.82
        Entity's Block location: World: (558,69,-216), Chunk: (at 14,4,8 in 34,-14; contains blocks 544,0,-224 to 559,255,-209), Re\
gion: (1,-1; contains chunks 32,-32 to 63,-1, blocks 512,0,-512 to 1023,255,-1)
        Entity's Momentum: 0.08, -0.08, -0.13
        Entity's Passengers: []
        Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
        at bcq.k_(SourceFile:825)
        at ur.k_(SourceFile:281)