diff --git a/src/Preferences.cc b/src/Preferences.cc
index 771a6f8c..73cbcfb8 100644
--- a/src/Preferences.cc
+++ b/src/Preferences.cc
@@ -144,6 +144,7 @@ void Preferences::init() {
 Preferences::~Preferences()
 {
 	removeDefaultSettings();
+	instance = NULL;
 }
 
 /**
@@ -501,15 +502,19 @@ void Preferences::apply() const
 	emit syntaxHighlightChanged(getValue("editor/syntaxhighlight").toString());
 }
 
-void Preferences::create(QWidget *parent, QStringList colorSchemes)
+void Preferences::create(QStringList colorSchemes)
 {
+    if (instance != NULL) {
+	return;
+    }
+
     std::list<std::string> names = ColorMap::inst()->colorSchemeNames(true);
     QStringList renderColorSchemes;
     foreach (std::string name, names) {
 	renderColorSchemes << name.c_str();
     }
     
-    instance = new Preferences(parent);
+    instance = new Preferences();
     instance->syntaxHighlight->clear();
     instance->syntaxHighlight->addItems(colorSchemes);
     instance->colorSchemeChooser->clear();
diff --git a/src/Preferences.h b/src/Preferences.h
index a40e92cd..0f862eb2 100644
--- a/src/Preferences.h
+++ b/src/Preferences.h
@@ -11,7 +11,7 @@ class Preferences : public QMainWindow, public Ui::Preferences
 public:
 	~Preferences();
         
-        static void create(QWidget *parent, QStringList colorSchemes);
+        static void create(QStringList colorSchemes);
 	static Preferences *inst();
 
 	QVariant getValue(const QString &key) const;
diff --git a/src/mainwin.cc b/src/mainwin.cc
index be3c4337..776a46ed 100644
--- a/src/mainwin.cc
+++ b/src/mainwin.cc
@@ -201,7 +201,7 @@ MainWindow::MainWindow(const QString &filename)
 #endif
 	    editor = new LegacyEditor(editorDockContents);
 
-	Preferences::create(this, editor->colorSchemes());
+	Preferences::create(editor->colorSchemes());
 
 	editorDockContents->layout()->addWidget(editor);
 
diff --git a/src/openscad.cc b/src/openscad.cc
index 80c1b049..bbd6c265 100644
--- a/src/openscad.cc
+++ b/src/openscad.cc
@@ -625,13 +625,14 @@ int gui(vector<string> &inputFiles, const fs::path &original_path, int argc, cha
 	}
 
 	MainWindow *mainwin;
-#ifdef ENABLE_MDI
-	BOOST_FOREACH(const string &infile, inputFiles) {
-		mainwin = new MainWindow(assemblePath(original_path, infile));
+	bool isMdi = settings.value("advanced/mdi", true).toBool();
+	if (isMdi) {
+	    BOOST_FOREACH(const string &infile, inputFiles) {
+		    mainwin = new MainWindow(assemblePath(original_path, infile));
+	    }
+	} else {
+	    mainwin = new MainWindow(assemblePath(original_path, inputFiles[0]));
 	}
-#else
-	mainwin = new MainWindow(assemblePath(original_path, inputFiles[0]));
-#endif
 
 	app.connect(&app, SIGNAL(lastWindowClosed()), &app, SLOT(quit()));
 	int rc = app.exec();
@@ -782,11 +783,6 @@ int main(int argc, char **argv)
 	if (vm.count("input-file"))	{
 		inputFiles = vm["input-file"].as<vector<string> >();
 	}
-#ifndef ENABLE_MDI
-	if (inputFiles.size() > 1) {
-		help(argv[0]);
-	}
-#endif
 
 	if (vm.count("colorscheme")) {
 		arg_colorscheme = vm["colorscheme"].as<string>();
@@ -807,6 +803,9 @@ int main(int argc, char **argv)
 	}
 
 	if (arg_info || cmdlinemode) {
+		if (inputFiles.size() > 1) {
+			help(argv[0]);
+		}
 		rc = cmdline(deps_output_file, inputFiles[0], camera, output_file, original_path, renderer, argc, argv);
 	}
 	else if (QtUseGUI()) {