This is very commonly happening on a SMP server.

{{-- Entity being ticked –
Details:
Entity Type: minecraft:drowned (ajg)
Entity ID: 128
Entity Name: Drowned
Entity's Exact location: -179.30, 56.48, -154.27
Entity's Block location: World: (-180,56,-155), Chunk: (at 12,3,5 in -12,-10; contains blocks -192,0,-160 to -177,255,-145), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
Entity's Momentum: 0.00, -0.09, -0.02
Entity's Passengers: []
Entity's Vehicle: ~_ERROR~ NullPointerException: null
Stacktrace:
at aug.p_(SourceFile:1475)
at se.p_(SourceFile:538)
}}