Fix editor crash cause by EditorFileSystem::get_singleton access in theme initialization path

editor/file_system/editor_file_system.cpp

@@ -3442,7 +3442,7 @@ bool EditorFileSystem::_should_skip_directory(const String &p_path) {
 
 	if (FileAccess::exists(p_path.path_join("project.godot"))) {
 		// Skip if another project inside this.
-		if (EditorFileSystem::get_singleton()->first_scan) {
+		if (EditorFileSystem::get_singleton() == nullptr || EditorFileSystem::get_singleton()->first_scan) {
 			WARN_PRINT_ONCE(vformat("Detected another project.godot at %s. The folder will be ignored.", p_path));
 		}
 		return true;