Crash in UsdImaging when "moving" an instanced primitive to a different scene graph location

[marktucker]

Description of Issue

After loading a particular USD file with a payload that contains an instanceable reference, updating the scene to load the same payload to a different scene graph location leads to a crash in the instancerAdapter.cpp in UsdImaging.

Steps to Reproduce

1. Load testA.usda into usdview from the attached zip file:
   instanceableCrash.zip
2. In the Interpreter window, run the following commands:
   l = Sdf.Layer.FindOrOpen("./testB.usda")
   usdviewApi.stage.GetRootLayer().TransferContent(l)
3. usdview will crash in UsdImagingInstanceAdapter::_Populate calling _GetPrim(instancerPath).IsInMaster() (because the _GetPrim call returns a null UsdPrim).

Package Versions

Approximately USD 20.05 (https://github.com/sideeffects/USD/tree/dev_houdini18.5.197, which is 20.05 plus a few additional changes, most or all of which I believe are in the USD dev branch already).

[c64kernal]

Filed as internal issue #USD-6146

[c64kernal]

We have a fix for this checked in and should be included in the 20.08 release.