EMSUSD-40 prevent editing instance proxy.

USD will refuse anyway, so better to prevent showing the option to the user.

lib/mayaUsd/fileio/primUpdaterManager.cpp

@@ -1124,6 +1124,11 @@ bool PrimUpdaterManager::editAsMaya(const Ufe::Path& path, const VtDictionary& u
         return false;
     }
 
+    if (pulledPrim.IsInstanceProxy()) {
+        TF_WARN("Cannot edit a USD instance proxy.");
+        return false;
+    }
+
     MayaUsd::ProgressBarScope progressBar(7, "Converting to Maya Data");
 
     PushPullScope scopeIt(_inPushPull);
@@ -1211,6 +1216,11 @@ bool PrimUpdaterManager::canEditAsMaya(const Ufe::Path& path) const
         return false;
     }
 
+    // USD refuses that we modify point instance proxies, so detect that.
+    if (prim.IsInstanceProxy()) {
+        return false;
+    }
+
     VtDictionary              userArgs;
     UsdMayaPrimUpdaterContext context(UsdTimeCode::Default(), prim.GetStage(), userArgs);