Was working on setting up the Animator and I deleted a parameter. Afterwards, I keep getting this error and I can't add any more parameters or interact correctly with the Animator Window. Undo did not get the error to go away.
Issue goes away by re-opening the Animator window.

Repro steps:
- open the simple project I created (repro.zip)
- open the SampleScene, click on the cube object and select a transition. You'll notice the parameter OpenDoor on it. Leave the Inspector open on that view.
- In Visual Studio, try to change the Line 21: if (Input.GetKeyDown(KeyCode.C)) (to say Keycode.A)
- save and switch back into the project
- don't wait for Unity to compile, just right-click on the parameter a few times and attempt to delete it (you don't actually have to delete it, it WILL disappear from the list)

1st error (appears once):
NullReferenceException:Object reference not set to an instance of an object
UnityEditor.DockArea:OnEnable()

2nd error (appears once):
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Graphs.AnimationStateMachine.GraphGUI.SyncGraphToUnitySelection (Boolean force) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/GraphGUI.cs:310)
UnityEditor.Graphs.AnimatorControllerTool.DetectAnimatorControllerFromSelection () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:730)
UnityEditor.Graphs.AnimatorControllerTool.OnEnable () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:743)

3rd error (appears every time I interract with Animator window):
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Graphs.ParameterControllerView.AddParameterMenu (System.Object value) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/ParameterControllerView.cs:584)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, Int32 selected) (at C:/buildslave/unity/build/Editor/Mono/GUI/GenericMenu.cs:111)

Previously tested on 2017.30f3 and I can't get it to reproduce.

There is a screenshot and a gif attached too.