Project Manager: Fix hacky code for project rename

Instantiating a new ProjectSettings is *not* the way to go.
ConfigFile works just fine to read/change a single value.

Fixes memory leaks as the instantiated ProjectSettings was never freed.
Forbid doing this to prevent such problems.

Fixes godotengine#25661.

core/config/project_settings.cpp

@@ -1200,6 +1200,7 @@ ProjectSettings::ProjectSettings() {
 	// Initialization of engine variables should be done in the setup() method,
 	// so that the values can be overridden from project.godot or project.binary.
 
+	CRASH_COND_MSG(singleton != nullptr, "Instantiating a new ProjectSettings singleton is not supported.");
 	singleton = this;
 
 	GLOBAL_DEF_BASIC("application/config/name", "");

editor/project_manager.cpp

@@ -436,17 +436,17 @@ class ProjectDialog : public ConfirmationDialog {
 				return;
 			}
 
-			ProjectSettings *current = memnew(ProjectSettings);
-
-			int err = current->setup(dir2, "");
+			// Load project.godot as ConfigFile to set the new name.
+			ConfigFile cfg;
+			String project_godot = dir2.path_join("project.godot");
+			Error err = cfg.load(project_godot);
 			if (err != OK) {
-				set_message(vformat(TTR("Couldn't load project.godot in project path (error %d). It may be missing or corrupted."), err), MESSAGE_ERROR);
+				set_message(vformat(TTR("Couldn't load project at '%s' (error %d). It may be missing or corrupted."), project_godot, err), MESSAGE_ERROR);
 			} else {
-				ProjectSettings::CustomMap edited_settings;
-				edited_settings["application/config/name"] = project_name->get_text().strip_edges();
-
-				if (current->save_custom(dir2.path_join("project.godot"), edited_settings, Vector<String>(), true) != OK) {
-					set_message(TTR("Couldn't edit project.godot in project path."), MESSAGE_ERROR);
+				cfg.set_value("application", "config/name", project_name->get_text().strip_edges());
+				err = cfg.save(project_godot);
+				if (err != OK) {
+					set_message(vformat(TTR("Couldn't save project at '%s' (error %d)."), project_godot, err), MESSAGE_ERROR);
 				}
 			}
 
@@ -694,18 +694,19 @@ class ProjectDialog : public ConfirmationDialog {
 			default_files_container->hide();
 			get_ok_button()->set_disabled(false);
 
-			ProjectSettings *current = memnew(ProjectSettings);
-
-			int err = current->setup(project_path->get_text(), "");
+			// Fetch current name from project.godot to prefill the text input.
+			ConfigFile cfg;
+			String project_godot = project_path->get_text().path_join("project.godot");
+			Error err = cfg.load(project_godot);
 			if (err != OK) {
-				set_message(vformat(TTR("Couldn't load project.godot in project path (error %d). It may be missing or corrupted."), err), MESSAGE_ERROR);
+				set_message(vformat(TTR("Couldn't load project at '%s' (error %d). It may be missing or corrupted."), project_godot, err), MESSAGE_ERROR);
 				status_rect->show();
 				msg->show();
 				get_ok_button()->set_disabled(true);
-			} else if (current->has_setting("application/config/name")) {
-				String proj = current->get("application/config/name");
-				project_name->set_text(proj);
-				_text_changed(proj);
+			} else {
+				String cur_name = cfg.get_value("application", "config/name", "");
+				project_name->set_text(cur_name);
+				_text_changed(cur_name);
 			}
 
 			project_name->call_deferred(SNAME("grab_focus"));