EMSUSD-305 Sets Add A Reference as default

lib/mayaUsd/resources/scripts/mayaUsdLibRegisterStrings.py

@@ -59,9 +59,9 @@ def mayaUsdLibRegisterStrings():
     register('kCacheMayaRefUsdHierarchy', 'Author Cache File to USD')
     register('kCaptionCacheToUsd', 'Cache to USD')
     register('kErrorCacheToUsdFailed', 'Cache to USD failed for "^1s".')
+    register('kMenuPrepend', 'Prepend')
     register('kMenuAppend', 'Append')
     register('kMenuPayload', 'Payload')
-    register('kMenuPrepend', 'Prepend (default)')
     register('kMenuReference', 'Reference')
     register('kOptionAsUSDReference', 'Composition Arc:')
     register('kOptionAsUSDReferenceToolTip', '<p>Choose the type of USD Reference composition arc for your Maya Reference:<br><br><b>Payloads</b> are a type of reference. They are recorded, but not traversed in the scene hierarchy. Select this arc if your goal is to manually construct<br>a "working set" that is a subset of an entire scene, in which only parts of the scene are required/loaded. Note: payloads are<br>weaker than direct references in any given LayerStack.<br><br><b>References</b> are general and can be used to compose smaller units of scene description into larger aggregates, building up a namespace that<br>includes the "encapsulated" result of composing the scene description targeted by a reference. Select this arc if your goal is not to unload your<br>references.</p>')

lib/mayaUsd/resources/scripts/mayaUsdUtils.py

@@ -189,7 +189,7 @@ def setUserSelectedUSDDialogFileFilter(fileFilter):
 
 def wantReferenceCompositionArc():
     opVarName = "mayaUsd_WantReferenceCompositionArc"
-    return cmds.optionVar(exists=opVarName) and cmds.optionVar(query=opVarName)
+    return not cmds.optionVar(exists=opVarName) or cmds.optionVar(query=opVarName)
 
 def saveWantReferenceCompositionArc(want):
     opVarName = "mayaUsd_WantReferenceCompositionArc"