when i summoned a bunch of mobs i tried to kill all them but then the game crashes for "ticking entity". same thing happens when i try to attack a baby polar bear. whenever i try to load my world, the game crashes stating that "game crashed whilst ticking entity." At least i have the crash log. Heres the crash log: 

---- Minecraft Crash Report -------- Minecraft Crash Report ----// Shall we play a game?
Time: 1/27/19 2:59 PMDescription: Ticking entity
java.lang.NullPointerException: Ticking entity at alb.g(SourceFile:81) at amv$c.c(SourceFile:219) at ajj.a(SourceFile:102) at ahm.cT(SourceFile:604) at ahl.k(SourceFile:2045) at ahm.k(SourceFile:473) at agz.k(SourceFile:134) at ami.k(SourceFile:47) at ahl.h(SourceFile:1896) at ahm.h(SourceFile:284) at amv.h(SourceFile:148) at bcl.a(SourceFile:958) at ur.a(SourceFile:363) at bcl.g(SourceFile:938) at bcl.k_(SourceFile:820) at ur.k_(SourceFile:279) at net.minecraft.server.MinecraftServer.b(SourceFile:862) at net.minecraft.server.MinecraftServer.a(SourceFile:791) at dri.a(SourceFile:124) at net.minecraft.server.MinecraftServer.run(SourceFile:657) at java.lang.Thread.run(Thread.java:745)

A detailed walkthrough of the error, its code path and all known details is as follows:---------------------------------------------------------------------------------------
– Head --Thread: Server threadStacktrace: at alb.g(SourceFile:81) at amv$c.c(SourceFile:219) at ajj.a(SourceFile:102) at ahm.cT(SourceFile:604) at ahl.k(SourceFile:2045) at ahm.k(SourceFile:473) at agz.k(SourceFile:134) at ami.k(SourceFile:47) at ahl.h(SourceFile:1896) at ahm.h(SourceFile:284) at amv.h(SourceFile:148) at bcl.a(SourceFile:958) at ur.a(SourceFile:363) at bcl.g(SourceFile:938)
– Entity being ticked --Details: Entity Type: minecraft:polar_bear (amv) Entity ID: 467 Entity Name: Polar Bear Entity's Exact location: 59.01, 56.64, 139.93 Entity's Block location: World: (59,56,139), Chunk: (at 11,3,11 in 3,8; contains blocks 48,0,128 to 63,255,143), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511) Entity's Momentum: 0.00, 0.19, 0.00 Entity's Passengers: [] Entity's Vehicle: ~_ERROR~ NullPointerException: nullStacktrace: at bcl.k_(SourceFile:820) at ur.k_(SourceFile:279)
– Affected level --Details: Level name: Entertainment world All players: 0 total; [] Chunk stats: ServerChunkCache: 2331 Level seed: -5131540176037800572 Level generator: ID 01 - flat, ver 0. Features enabled: true Level generator options: {biome:"minecraft:desert",layers:[

,{block:"minecraft:stone",height:3},{block:"minecraft:sandstone",height:52}],structures:{}} Level spawn location: World: (8,55,8), Chunk: (at 8,3,8 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511) Level time: 15950345 game time, 1000 day time Level dimension: 0 Level storage version: 0x04ABD - Anvil Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false) Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: trueStacktrace: at net.minecraft.server.MinecraftServer.b(SourceFile:862) at net.minecraft.server.MinecraftServer.a(SourceFile:791) at dri.a(SourceFile:124) at net.minecraft.server.MinecraftServer.run(SourceFile:657) at java.lang.Thread.run(Thread.java:745)
– System Details --Details: Minecraft Version: 19w04b Operating System: Windows 10 (amd64) version 10.0 Java Version: 1.8.0_51, Oracle Corporation Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation Memory: 388427360 bytes (370 MB) / 1778384896 bytes (1696 MB) up to 2147483648 bytes (2048 MB) JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M Player Count: 0 / 8; [] Data Packs: vanilla Type: Integrated Server (map_client.txt) Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.