do not open dominant script if external editor is selected, fixes #3067

tools/editor/plugins/script_editor_plugin.cpp

@@ -2256,6 +2256,9 @@ void ScriptEditor::_history_back(){
 void ScriptEditor::set_scene_root_script( Ref<Script> p_script ) {
 
 	bool open_dominant = EditorSettings::get_singleton()->get("text_editor/open_dominant_script_on_scene_change");
+	if (bool(EditorSettings::get_singleton()->get("external_editor/use_external_editor")))
+		return;
+
 	if (open_dominant && p_script.is_valid()) {
 		edit(p_script);
 	}