(v2.6.3) When you manually create a new encounter in Combat Tracker, you receive the error.
Uncaught (in promise) TypeError: Cannot read property '_id' of undefined
at turnmarker.js:38
at Function._call (foundry.js:2210)
at Function.callAll (foundry.js:2170)
at foundry.js:9697
at Array.map (<anonymous>)
at Function._handleUpdate (foundry.js:9688)
at Function.update (foundry.js:9669)
at async CombatTracker._onCombatCreate (foundry.js:40092)
Nothing is exploding, though.