I created nbt and forgotten to setSkin
and now server crashing with error
This is a plugin issue. If you create a non-player human entity without a skin, this exception will be raised.
Is possible to fix it without deleting world?
You should be able to use an NBT editor to remove the invalid entity.
Ok, I have tried NBT Editors,but I have here only this

What I must edit? I think there is not something with amount of entities etc
Shouldn't the core prevent the entity from being saved in the first place?
This exception should be thrown when actually _constructing_ the entity, not when trying to spawn it. Currently the entity is perfectly "valid" and will be loaded and saved despite the fact it has no skin. The crash will be triggered when the server attempts to spawn the bad entity _to_ a player.
I'm working on a solution for this following what @dktapps said
The referenced pull request resolves this issue, the issue will be closed automatically when changes are merged to master. Leaving this open in case anyone encounters the same issue with other branches.
Most helpful comment
Shouldn't the core prevent the entity from being saved in the first place?