Fix prim expiry crashes and add context menu hierarchy ops

Crash fixes (expired UsdPrim after namespace edits):
- RenderPrimNode: early-out after rename commit (renamed flag + column cleanup)
- RenderPrimNode: early-out after context menu op expires prim (prim.IsValid() guard)
- RenderContextMenu: EndPopup+return when Unparent/Group expires prim mid-popup
- Child iteration: snapshot GetChildren() into vector before recursing to guard
  against live iterator invalidation from stage mutations deeper in the tree
- Rename: firstFrame guard prevents spurious IsItemDeactivated cancel on
  SetKeyboardFocusHere activation frame (fixes second-rename not working)
- RenamePrimCommand: check RenamePrim() return value, log if rejected

New features:
- Context menu Add Child submenu: create typed child prim under selection
- Context menu Unparent to Root: move prim to stage root (undo-able)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/core/commands/RenamePrimCommand.cpp

@@ -27,7 +27,10 @@ void RenamePrimCommand::Execute() {
         return;
     }
     UsdNamespaceEditor editor(m_stage);
-    editor.RenamePrim(prim, TfToken(m_newName));
+    if (!editor.RenamePrim(prim, TfToken(m_newName))) {
+        LOG_ERROR("RenamePrimCommand::Execute: RenamePrim() rejected for " + m_oldPath.GetString());
+        return;
+    }
     std::string whyNot;
     if (!editor.CanApplyEdits(&whyNot)) {
         LOG_ERROR("RenamePrimCommand::Execute: cannot rename " + m_oldPath.GetString() + ": " + whyNot);
@@ -45,7 +48,10 @@ void RenamePrimCommand::Undo() {
         return;
     }
     UsdNamespaceEditor editor(m_stage);
-    editor.RenamePrim(prim, TfToken(m_oldName));
+    if (!editor.RenamePrim(prim, TfToken(m_oldName))) {
+        LOG_ERROR("RenamePrimCommand::Undo: RenamePrim() rejected for " + m_newPath.GetString());
+        return;
+    }
     std::string whyNot;
     if (!editor.CanApplyEdits(&whyNot)) {
         LOG_ERROR("RenamePrimCommand::Undo: cannot rename back " + m_newPath.GetString() + ": " + whyNot);

src/ui/SceneHierarchyPanel.cpp

@@ -597,7 +597,8 @@ void SceneHierarchyPanel::RenderPrimNode(const UsdPrim& prim) {
     if (!hasChildren)  flags |= ImGuiTreeNodeFlags_Leaf |
                                 ImGuiTreeNodeFlags_NoTreePushOnOpen;
 
-    bool open = false;
+    bool open    = false;
+    bool renamed = false;   // true if rename committed this frame (prim now expired)
     if (isRenaming) {
         // Render just the arrow (hidden label) so we keep indentation and open/close,
         // then overlay an InputText inline for the prim name.
@@ -606,6 +607,9 @@ void SceneHierarchyPanel::RenderPrimNode(const UsdPrim& prim) {
             ImGui::PopStyleColor();
         ImGui::SameLine();
         ImGui::SetNextItemWidth(-1.0f);
+        // firstFrame: skip deactivation-cancel on the exact frame SetKeyboardFocusHere
+        // activates the widget — IsItemDeactivated() can fire spuriously on that frame.
+        bool firstFrame = m_renameJustStarted;
         if (m_renameJustStarted) {
             ImGui::SetKeyboardFocusHere();
             m_renameJustStarted = false;
@@ -613,7 +617,7 @@ void SceneHierarchyPanel::RenderPrimNode(const UsdPrim& prim) {
         bool commit = ImGui::InputText("##rename_input", m_renameBuf, sizeof(m_renameBuf),
                                        ImGuiInputTextFlags_EnterReturnsTrue |
                                        ImGuiInputTextFlags_AutoSelectAll);
-        bool canceled = ImGui::IsItemDeactivated() && !commit;
+        bool canceled = !firstFrame && ImGui::IsItemDeactivated() && !commit;
         if (commit && m_renameBuf[0] != '\0') {
             std::string newName = SanitizeUsdName(m_renameBuf);
             if (!newName.empty() && newName != displayName && m_commandHistory) {
@@ -621,6 +625,7 @@ void SceneHierarchyPanel::RenderPrimNode(const UsdPrim& prim) {
                     m_stage, primPath, newName));
                 SdfPath newPath = primPath.GetParentPath().AppendChild(TfToken(newName));
                 SetSelectedPathFromClick(newPath.GetString());
+                renamed = true;  // prim at primPath is now expired — stop here
             }
             m_renamingPath = SdfPath();
         }
@@ -632,6 +637,17 @@ void SceneHierarchyPanel::RenderPrimNode(const UsdPrim& prim) {
             ImGui::PopStyleColor();
     }
 
+    // ── Early-out after rename: prim is expired, close table row and return ──
+    if (renamed) {
+        ImGui::TableNextColumn();   // Col 1
+        ImGui::TableNextColumn();   // Col 2
+        ImGui::TableNextColumn();   // Col 3
+        if (open && hasChildren)
+            ImGui::TreePop();
+        ImGui::PopID();
+        return;
+    }
+
     // ── Scroll-to-selection (SpanAllColumns gives correct full-row rect) ────
     if (m_scrollToSelected && primStr == m_primarySelectedPath) {
         ImGui::SetScrollHereY(0.5f);
@@ -691,6 +707,18 @@ void SceneHierarchyPanel::RenderPrimNode(const UsdPrim& prim) {
 
         // Context menu (must follow the last widget = the tree node).
         RenderContextMenu(prim);
+
+        // A context menu op (Unparent, Group, etc.) may have moved this prim,
+        // expiring it. Bail out before touching the dead prim object.
+        if (!prim.IsValid()) {
+            ImGui::TableNextColumn();   // Col 1
+            ImGui::TableNextColumn();   // Col 2
+            ImGui::TableNextColumn();   // Col 3
+            if (open && hasChildren)
+                ImGui::TreePop();
+            ImGui::PopID();
+            return;
+        }
     }
 
     // ── Col 1: Prim-type icon ───────────────────────────────────────────────
@@ -769,7 +797,11 @@ void SceneHierarchyPanel::RenderPrimNode(const UsdPrim& prim) {
     // Solution: recurse here (after columns), but ImGui only needs TreePop to
     // be inside the same Begin/End pair — column doesn't matter for TreePop.
     if (open && hasChildren) {
-        for (const auto& child : prim.GetChildren())
+        // Snapshot children: stage mutations inside RenderPrimNode (reparent, group)
+        // invalidate the live GetChildren() iterator.
+        std::vector<UsdPrim> children(prim.GetChildren().begin(),
+                                      prim.GetChildren().end());
+        for (const auto& child : children)
             RenderPrimNode(child);
         ImGui::TreePop();
     }
@@ -817,6 +849,44 @@ void SceneHierarchyPanel::RenderContextMenu(const UsdPrim& prim) {
 
         ImGui::Separator();
 
+        // ── Add Child Prim ────────────────────────────────────────────────────
+        static const char* kChildPrimTypes[] = {
+            "Xform", "Scope",
+            "Mesh", "Sphere", "Cube", "Cylinder", "Cone", "Capsule",
+            "Camera",
+            "SphereLight", "DomeLight", "RectLight", "DiskLight",
+            "CylinderLight", "DistantLight"
+        };
+        if (ImGui::BeginMenu("Add Child")) {
+            SdfPath parentPath = prim.GetPath();
+            for (const char* typeName : kChildPrimTypes) {
+                if (ImGui::MenuItem(typeName)) {
+                    std::string baseName = typeName;
+                    SdfPath childPath = FindUniqueChildPath(parentPath, baseName);
+                    if (m_commandHistory) {
+                        m_commandHistory->Push(std::make_unique<CreatePrimCommand>(
+                            m_stage, childPath, TfToken(typeName)));
+                        UsdPrim child = m_stage->GetPrimAtPath(childPath);
+                        if (child.IsValid()) {
+                            SetSelectedPathFromClick(childPath.GetString());
+                            m_scrollToSelected = true;
+                        }
+                    } else {
+                        UsdPrim child = m_stage->DefinePrim(childPath, TfToken(typeName));
+                        if (child.IsValid()) {
+                            SetSelectedPathFromClick(childPath.GetString());
+                            m_scrollToSelected = true;
+                        } else {
+                            LOG_ERROR("Failed to create child prim: " + childPath.GetString());
+                        }
+                    }
+                }
+            }
+            ImGui::EndMenu();
+        }
+
+        ImGui::Separator();
+
         // ── Rename / Group ────────────────────────────────────────────────────
         if (ImGui::MenuItem("Rename", "F2")) {
             m_renamingPath = prim.GetPath();
@@ -826,6 +896,15 @@ void SceneHierarchyPanel::RenderContextMenu(const UsdPrim& prim) {
             m_renameJustStarted = true;
             m_scrollToSelected  = true;
         }
+        bool canUnparent = (prim.GetPath().GetPathElementCount() > 1);
+        if (ImGui::MenuItem("Unparent to Root", nullptr, false, canUnparent)) {
+            if (m_commandHistory && m_stage) {
+                std::string name = FindUniqueChildPath(SdfPath("/"), prim.GetName().GetString()).GetName();
+                m_commandHistory->Push(std::make_unique<ReparentPrimCommand>(
+                    m_stage, prim.GetPath(), SdfPath("/"), name));
+                SetSelectedPathFromClick(SdfPath("/").AppendChild(TfToken(name)).GetString());
+            }
+        }
         if (ImGui::MenuItem("Group", "Ctrl+G")) {
             std::vector<SdfPath> srcs;
             if (m_selectedPaths.count(prim.GetPath().GetString()))
@@ -852,6 +931,13 @@ void SceneHierarchyPanel::RenderContextMenu(const UsdPrim& prim) {
             }
         }
 
+        // Unparent or Group may have moved this prim, expiring it.
+        // Close the popup now before touching any more prim state.
+        if (!prim.IsValid()) {
+            ImGui::EndPopup();
+            return;
+        }
+
         ImGui::Separator();
 
         bool hasChildren = !prim.GetChildren().empty();