Writing Editor Script, Getting Weird Null Reference Exception

I am writing a custom editor that will allow the user to add items to a list and print
labels in the inspector for each item.

I declare the list as follows:

public List<string> StageTable;

A bit later in the code I have this piece of code to create a label for each string in

foreach(string st in StageTable)
			EditorGUILayout.LabelField("test string");

I can see the result in the inspector, but when I try the game it blanks out the inspector
where the result should be showing nothing there, and throws this long exception:

Object reference not set to an instance of an object
() (at Assets/Editor/APG_CustomEditor.cs:53)
(Boolean isRepaintEvent, UnityEditor.Editor[]
editors, Boolean eyeDropperDirty)
(at C:/BuildAgent/work/14194e8ce88cdf47/Editor/Mono/ ...
UnityEditor.InspectorWindow.OnGUI ()
(at C:/BuildAgent/work/14194e8ce88cdf47/Editor/Mono/ ...
System.Reflection.MonoMethod.Invoke (System.Object obj,
BindingFlags invokeAttr, System.Reflection.Binder binder,
System.Object[] parameters,
System.Globalization.CultureInfo culture)

Any help or input on this matter is of course, most welcome!

I think that the StageTable is not initialized when you try to built the LabelField for it. Null list and blank list is different, the first will produce nullref-exception and the second will be okay (just that there is no labelfield to be shown)