Steps to reproduce:
1. Download and open the attached project
2 Open window at Window->Humanoid Animation Setting Window.
3. Set a unitychanAvatar in Assets/UnityChan/unitychan at Rig:Avatar Definition.
4. Set a Assets/UnityChan/AvatarMaskAll at Animations:Mask.
5. Select Assets/UnityChan/unitychan_JUMP00 in Project window
6. Click "Set Selected fbx" in Humanoid Animation Setting Window.

Expected result: fbx is set
Actual result: fbx is not set, error is thrown

Assertion failed on expression: '!((transfer.IsReading() || transfer.IsWriting()) && (temp != 0 && temp != 1))'
UnityEditor.AssetImporter:SaveAndReimport()
App.HumanoidAnimationSettingWindow:SetAnimationImporterSettings(ModelImporter) (at Assets/Scripts/Editor/HumanoidAnimationSettingWindow.cs:76)
App.HumanoidAnimationSettingWindow:SetAnimationOptions() (at Assets/Scripts/Editor/HumanoidAnimationSettingWindow.cs:93)
App.HumanoidAnimationSettingWindow:OnGUI() (at Assets/Scripts/Editor/HumanoidAnimationSettingWindow.cs:49)
UnityEditor.DockArea:OnGUI()

Reproduced on: 5.5.0b4, 5.5.0b6, 5.5.0b11, 5.5.0f3, 5.5.0p4, 5.6.0b4
Not reproduced on: 5.4.4f1, 5.5.0b1,5.5.0b2, 5.5.0b3

Notes:
-jumping between versions sometimes makes the issue unreproducible