Fix NbWindow crash

Turns out passing null as the first argument to
NbWindow.createObject is an error. This was so simple in hindsight
I don't know why it took me so long to find.
diff --git a/ide/qml/main.qml b/ide/qml/main.qml
index 210640c..4c115d5 100644
--- a/ide/qml/main.qml
+++ b/ide/qml/main.qml
@@ -26,7 +26,7 @@
 
     function openNotebook(path=null) {
         let NbWindow = Qt.createComponent("qrc:///qml/NbWindow.qml");
-        let window = NbWindow.createObject(null, {welcomeWindow: root});
+        let window = NbWindow.createObject(root, {welcomeWindow: root});
 
         if (path !== null)
         {