dbSta: fix STA cleanup for database reload

Clear staCell pointers on all masters before freeing the network to
prevent the GUI render thread from accessing freed ConcreteCell objects.
Recreate the default scene after deleteScenes() so cmd_scene_ remains
valid for heatmap settings queries on exit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Eder Monteiro <emrmonteiro@precisioninno.com>

Author: eder-matheus

src/dbSta/src/dbSta.cc

@@ -326,9 +326,18 @@ void dbSta::postReadDb(odb::dbDatabase* db)
 void dbSta::preDbClear(odb::dbDatabase* db)
 {
   db_cbk_->removeOwner();
+  // Clear staCell pointers on all masters before freeing the network.
+  // The GUI render thread may still be accessing these concurrently.
+  for (odb::dbLib* lib : db->getLibs()) {
+    for (odb::dbMaster* master : lib->getMasters()) {
+      master->staSetCell(nullptr);
+    }
+  }
   // Delete scenes before clearing the network, since scenes hold
   // LibertyLibrary pointers that ConcreteNetwork::clear() will free.
+  // Recreate the default scene so cmd_scene_ remains valid.
   deleteScenes();
+  makeDefaultScene();
   clear();
   db_network_->clear();
   // Restore db_ since dbNetwork::clear() nulls it, but we need it for reload