When someone leaves a team, the team will became unfunctional.
The team still exists, appears within "/team list" and entities can join and be targeted (team=<team>).
However, the team doesn't shows within "/team join" or "/team modify" auto-complete and teams properties (color / collision rule) doesn't work.

How to reproduce:

• Create a dummy team
  /team add dummy
• Modify one of the properties of the team
  /team modify dummy color aqua
• Make you and an entity join the team
  /team join dummy @s
  /team join dummy @e[type=!player,limit=1]
• Remove the entity from the team
  /team leave @e[type=!player,team=dummy]
• The team is now unusable

Console output when joining/leaving the team:

Error executing task on Client
java.lang.NullPointerException
	at dkv.a(SourceFile:220)
	at efc.a(SourceFile:2076)
	at sk.a(SourceFile:130)
	at sk.a(SourceFile:15)
	at pd.a(SourceFile:21)
	at pd$$Lambda$3978/15655054.run(Unknown Source)
	at apq.c(SourceFile:144)
	at apu.c(SourceFile:23)
	at apq.y(SourceFile:118)
	at apq.bl(SourceFile:103)
	at drn.e(SourceFile:1019)
	at drn.e(SourceFile:688)
	at net.minecraft.client.main.Main.main(SourceFile:215)