While trying to make a (rather complex) editor for a component, I decided that, instead of trying to cram everything into a custom inspector, to just make a simple one that creates a custom EditorWindow, like this:

function OnInspectorGUI () {
 if (GUILayout.Button("Open Dialogue Editor")) {
   var window:DialogueEditor = EditorWindow.GetWindow(DialogueEditor);
      window.SetTarget(target);
      window.Init();
 }
}

And use this window to edit the target. However, if the target is deleted and the window is still open, it should close itself. For that purpose I wrote the following at the start of the window's OnGUI():

if (!target) {
  this.Close(); 
  return;
}

However, if I test it out by simply not setting the target, I get the following errors:

NullReferenceException: CalculateNextFromHintList accessing GUI state from destroyed monobehaviour
UnityEngine.IDList.GetNext (Int32 hint, FocusType focusType, Rect position) (at C:/BuildAgent/work/14194e8ce88cdf47/Runtime/ExportGenerated/Editor/GUIUtility.cs:66)
UnityEngine.IDList.GetNext (Int32 hint, FocusType focusType) (at C:/BuildAgent/work/14194e8ce88cdf47/Runtime/ExportGenerated/Editor/GUIUtility.cs:59)
UnityEngine.GUIUtility.GetControlID (Int32 hint, FocusType focus) (at C:/BuildAgent/work/14194e8ce88cdf47/Runtime/ExportGenerated/Editor/GUIUtility.cs:254)
UnityEngine.GUI.Box (Rect position, UnityEngine.GUIContent content, UnityEngine.GUIStyle style) (at C:/BuildAgent/work/14194e8ce88cdf47/Runtime/ExportGenerated/Editor/GUI.cs:215)
UnityEditor.DockArea.OnGUI () (at C:/BuildAgent/work/14194e8ce88cdf47/Editor/Mono/GUI/DockArea.cs:664)

And the slightly more intuitive

GUI Window tries to begin rendering while something else has not finished rendering! Either you have a recursive OnGUI rendering, or previous OnGUI did not clean up properly.

Am I doing something wrong? This seems to be all fine according to the Reference and the examples they've given.