Testing command blocks letting zombies summon fireballs and crashed suddenly.

Reported as following info. Later I tried to trigger this bug again but failed.

Description: Ticking entity

java.lang.NullPointerException: Cannot invoke "deu.j()" because "$$0" is null
    at cvx.<init>(SourceFile:268)
    at cvx.<init>(SourceFile:252)
    at cqv.b(SourceFile:70)
    at cqv.a(SourceFile:107)
    at cqv.a(SourceFile:46)
    at dfk.l(SourceFile:201)
    at dfk.h(SourceFile:256)
    at arm.a(SourceFile:1091)
    at dev.a(SourceFile:552)
    at coz.a(SourceFile:34)
    at cpb.b(SourceFile:241)
    at coq.l(SourceFile:98)
    at arm.a(SourceFile:760)
    at dev.a(SourceFile:503)
    at arm.a(SourceFile:405)
    at dzn.a(SourceFile:54)
    at arm.a(SourceFile:375)
    at net.minecraft.server.MinecraftServer.c(SourceFile:1057)
    at net.minecraft.server.MinecraftServer.a(SourceFile:944)
    at hcd.a(SourceFile:113)
    at net.minecraft.server.MinecraftServer.y(SourceFile:707)
    at net.minecraft.server.MinecraftServer.a(SourceFile:289)
    at java.base/java.lang.Thread.run(Thread.java:1583)

– Entity being ticked –
Details:
    Entity Type: minecraft:fireball (coz)
    Entity ID: 3201
    Entity Name: 鐏悆
    Entity's Exact location: 46.32, 63.00, -4.47
    Entity's Block location: World: (46,63,-5), Section: (at 14,15,11 in 2,3,-1; chunk contains blocks 32,-64,-16 to 47,319,-1), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,-64,-512 to 511,319,-1)
    Entity's Momentum: 0.00, 0.00, -0.08
    Entity's Passengers: []
    Entity's Vehicle: null
Stacktrace:
    at dev.a(SourceFile:503)
    at arm.a(SourceFile:405)
    at dzn.a(SourceFile:54)
    at arm.a(SourceFile:375)
    at net.minecraft.server.MinecraftServer.c(SourceFile:1057)
    at net.minecraft.server.MinecraftServer.a(SourceFile:944)
    at hcd.a(SourceFile:113)
    at net.minecraft.server.MinecraftServer.y(SourceFile:707)
    at net.minecraft.server.MinecraftServer.a(SourceFile:289)
    at java.base/java.lang.Thread.run(Thread.java:1583)