Copy From Other Avatar generates errors when the source avatar is not of the same type (Generic vs. Humanoid).

Repro:
-open project
-select motion file (Male_Idle_T1_Cut01_James)
-in Rig tab, set Avatar Definition to Copy From Other Avatar.
-from Project, drag TStance>TStanceAvatar into Source, click Apply and “Yes” to the pop-up for Humanoid…
Note: important to drag’n’drop avatar from Project window and not click Inspector browser button.
Bug: 2 errors in the console…

this is a regression.
Not repro in…
Version 2017.2.0p2 (dbc2eb12ac98)
Version 2017.3.0a7 (2bdce6e797f4)

Repro in…
Version 2017.3.0b1 (bc2668834c45)

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/Runtime/IMGUI/Managed/GUILayoutUtility.cs:296)
UnityEngine.GUILayout.BeginHorizontal (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:306)
UnityEngine.GUILayout.BeginHorizontal (UnityEngine.GUILayoutOption[] options) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:293)
UnityEditor.Experimental.AssetImporters.AssetImporterEditor.ApplyRevertGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/ImportSettings/AssetImporterEditor.cs:258)
UnityEditor.AssetImporterTabbedEditor.OnInspectorGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/TabbedEditor.cs:151)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[] editors, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1243)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

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.EndVertical () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayout.cs:342)
UnityEditor.EditorGUILayout.EndVertical () (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorGUI.cs:7837)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor[] editors, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1253)
UnityEditor.InspectorWindow.DrawEditors (UnityEditor.Editor[] editors) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1020)
UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:361)
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)

In Version 2018.1.0a6 (579e6ed209a9), errors are a bit different, but still present.