Originally reported on Github: [https://github.com/Unity-Technologies/usd-unity-sdk/issues/282]
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*Possibly *related tickets:
USDU-235 / USDU-236 / USDU-237 / USDU-238 / USDU-239
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Exporting an instanced prefab with materials into USD and then viewing it with {{usdview}} causes it to emit these error messages - and display the model with no material:
{code:java}

(USDtools) D:\python\usdmorph\HumanUsd\h64>usdview Human-0064-0.usda
Coding Error (secondary thread): in GetBehavior at line 540 of D:\transfer\USD\pxr\usd\usdShade\connectableAPIBehavior.cpp -- Could not find prim type '' for prim prim </Human_0064/Materials/Man001_color_28332/PreviewSurface> on stage with rootLayer @Human-0064-0.usda@, sessionLayer @anon:00000000295339F0@
... (4 more of the same)
Coding Error (secondary thread): in GetBehavior at line 540 of D:\transfer\USD\pxr\usd\usdShade\connectableAPIBehavior.cpp -- Could not find prim type '' for prim prim </Human_0064/Materials/Man001_color_28332/PreviewSurface> on stage with rootLayer @Human-0064-0.usda@, sessionLayer @anon:00000000295339F0@
ERROR: Usdview encountered an error while rendering.
Error in 'pxrInternal_v0_21__pxrReserved__::`anonymous-namespace'::_BehaviorRegistry::GetBehavior' at line 540 in file D:\transfer\USD\pxr\usd\usdShade\connectableAPIBehavior.cpp : 'Could not find prim type '' for prim prim </Human_0064/Materials/Man001_color_28332/PreviewSurface> on stage with rootLayer @Human-0064-0.usda@, sessionLayer @anon:00000000295339F0@'
... (6 more of the same)
Error in 'pxrInternal_v0_21__pxrReserved__::`anonymous-namespace'::_BehaviorRegistry::GetBehavior' at line 540 in file D:\transfer\USD\pxr\usd\usdShade\connectableAPIBehavior.cpp : 'Could not find prim type '' for prim prim </Human_0064/Materials/Man001_color_28332/PreviewSurface> on stage with rootLayer @Human-0064-0.usda@, sessionLayer @anon:00000000295339F0@'
{code}

The relevent USD lines are as follows:

{code:java}
 {{ def Scope "Materials"
{
def Material "Man001_color_28332"
{
token outputs:displacement
token outputs:surface.connect = </Human_0064/Materials/Man001_color_28332/PreviewSurface.outputs:surface>
token[] requiredKeywords

def "PreviewSurface"
{
token info:id = "UsdPreviewSurface"
float inputs:clearcoat = 0
float inputs:clearcoatRoughness = 0.01
float3 inputs:diffuseColor = (0.18, 0.18, 0.18)}}
The errors go away if I add a {{Shader}} class to the "PreviewSurface" line like so:
def Shader"PreviewSurface"
{
{code}

Notes:
* Using Unity 2020.3.20f1
* Using USD Package 3..0.0-exp.2
* A repo with Python code that does a minimal parsing of a USDA file and repairs the issues 278, 282, 283, 284, 285 can be found at [https://github.com/MikeWise2718/usdmorph] - hopefully it won't be needed long before these are all repaired in a new version.