Only enable delete menu item for root nodes

MongoView/View Controllers/QueryViewController.swift

@@ -173,7 +173,11 @@ class QueryViewController: NSViewController {
 extension QueryViewController: NSMenuItemValidation {
     func validateMenuItem(_ menuItem: NSMenuItem) -> Bool {
         if menuItem.action == #selector(deleteNode(_:)) {
-            return outlineView.clickedRow != -1
+            if outlineView.clickedRow != -1, let node = outlineView.item(atRow: outlineView.clickedRow) as? Node, node.parent == nil {
+                return true
+            } else {
+                return false
+            }
         }
         return true
     }