When deleting a clip from a blend tree in the Animator controller graph (select the clip node in blend tree & RMB > delete), no errors are generated as expected.
But, if you select the master blend tree (in this case Speed) in the graph, then select the Idle_A motion row and click the “-“ button in the Inspector… one or multiple errors are generated in the console, depending on the Unity version.

Repro:
-open the project
-open “loco” scene
-select Winston and open Animator window
-double-click on the Blend Tree node to enter it
-select the Speed blend tree node
-in the Inspector, select the Idle_A motion row and click the “-“ button to delete it…
Bug: error in the console
Expected: no errrors

Not repro in…
Version 2017.2.0p2 and…
Version 2017.3.0a7 (2bdce6e797f4)

Repro in…
In Version 2017.3.0b1 (bc2668834c45)

InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:323)
UnityEngine.GUILayout.EndScrollView (Boolean handleScrollWheel) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:455)
UnityEditor.EditorGUILayout.EndScrollView () (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorGUI.cs:8054)
UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:394)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:295)
UnityEditor.HostView.Invoke (System.String methodName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:288)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:255)

Starting from 2018.1.0a6 (4f1daad06e4d), the above error no longer exist, but the following error is generated instead…

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.GUILayoutUtility.BeginLayoutGroup (UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options, System.Type layoutType) (at /Users/builduser/buildslave/unity/build/Modules/IMGUI/GUILayoutUtility.cs:317)
UnityEditor.EditorGUILayout.BeginHorizontal (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7785)
UnityEditor.InspectorWindow.DrawPreviewAndLabels () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:681)
UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:449)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:295)
UnityEditor.HostView.Invoke (System.String methodName) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:288)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:255)