Fix Traceback when adding empty tab

Re #2435

spinetoolbox/spine_db_editor/widgets/multi_spine_db_editor.py

@@ -93,22 +93,23 @@ def _make_new_tab(self, db_url_codenames=None, window=False):  # pylint: disable
         if not tab.load_db_urls(db_url_codenames, create=True, window=window):
             return
         # Checks if the same url is already opened under a different codename
-        for url, codename in db_url_codenames.items():
-            current_codename = codename
-            for db_map in self.db_mngr.db_maps:
-                is_same = str(url) == db_map.db_url
-                is_same = is_same and codename != db_map.codename
-                if is_same:
-                    existing = db_map.codename
-                    msg = QMessageBox(self.parent())
-                    msg.setIcon(QMessageBox.Icon.Information)
-                    msg.setWindowTitle(current_codename)
-                    msg.setText(
-                        f"<p>The same database is already open in another window. Opening the new window under "
-                        f"the same name as the existing one: <b>{existing}</b></p>"
-                    )
-                    msg.exec()
-                    break
+        if db_url_codenames:
+            for url, codename in db_url_codenames.items():
+                current_codename = codename
+                for db_map in self.db_mngr.db_maps:
+                    is_same = str(url) == db_map.db_url
+                    is_same = is_same and codename != db_map.codename
+                    if is_same:
+                        existing = db_map.codename
+                        msg = QMessageBox(self.parent())
+                        msg.setIcon(QMessageBox.Icon.Information)
+                        msg.setWindowTitle(current_codename)
+                        msg.setText(
+                            f"<p>The same database is already open in another window. Opening the new window under "
+                            f"the same name as the existing one: <b>{existing}</b></p>"
+                        )
+                        msg.exec()
+                        break
         return tab
 
     def show_plus_button_context_menu(self, global_pos):