A customer asked me about infinity loop issue occurred at Particle System Editor.

Be occurring an infinity loop issue when Sub-Emitters sets between same hierarchy particles (They are not root object!!) in ParticleSystem Editor.

I think to cause of this commit.
https://github.cds.internal.unity3d.com/unity/unity/commit/7897dc5d540c34027bb84bca3e932e4f5e5d9188

| parent = parent.transform;

Because inputted a same transform value to parent variable, I think to can't out a loop.

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Step to rerpduce:
1. Creates a new project.
2. Adds a new particle effect A as root obejct in hierarchy
3. Add new two particle effects (B, C) under the A object.
4. Sets the C object at B's Sub-Emitters.

Actual Result:
The editor can't get out of 'while' loop statement and it come to an infinity loop state.

Expected Result:
EditorGUILayout.HelpBox(string.Format("{0}\n{1}", s_Texts.invalidSubEmitterParenting.text, invalidSubEmitters.TrimEnd('\n')), MessageType.Error, true);
This code is ran and an error message is displayed normally. And the editor doesn't come to an infinity loop state.

Reproducible in:
2020.2.0f1, 2020.2.1f1, 2020.2.2f1 (all version of above 2020.2.0f1)