Losing reference to propertyField after play and then return to editor mode

Hello,

I am creating a property field on CreateInspectorGUI() and attaching it to the root visual element.

everything works perfectly and the property fields shows normal; but when I play the scene and then get back to the editor, the property field disappears.

the test label I created still shows after play/get back to the editor.

Code (CSharp):

1.        
2.             private SerializedProperty dialogueShowUpperCaseProperty;
3.  
4.             dialogueShowUpperCaseProperty = serializedObject.FindProperty("showUpperCase");
5.             PropertyField showUpperField = new PropertyField(dialogueShowUpperCaseProperty);
6.             conversationSettingsHolder.Add(showUpperField);
7.  
8.             Label testLabel = new Label("test label");
9.             conversationSettingsHolder.Add(testLabel);
10.  
11.             root.Add(conversationSettingsHolder);

any ideas?

thanks