How to reproduce:
1. Create a new VFX graph
2. Delete 'Simple Particle System' from the graph and add 'Heads and Sparks' system to the graph
3. Save the graph
4. Create a new Graph and open it
5. Delete newly created Graph from project browser in order to get empty VFX window
6. Create a new VFX using 'Create a new Visual Effect Graph' button with the same name as first created VFX

Expected result: Empty VFX window is displayed as second graph intends to replace the first one
Actual result: First graph is visible and not replaced by the second one, errors are thrown to the console

Reproduced with: 2022.1.0a16 (master from 2021/11/22)

Notes:
- Seem to happen only when having multiple windows
- Error message:
InvalidOperationException: SystemNames : Model is not registered null
UnityEditor.VFX.VFXSystemNames.GetUniqueSystemName (UnityEditor.VFX.VFXModel model) (at com.unity.visualeffectgraph/Editor/Models/VFXSystemNames.cs:108)