EditorGUILayout.TextField changes back to first value I input [GIF of the problem inside]

When I input for example "aaa" in that text from the custom editor and then change it from another code. and then i click on the field again, my initial input "aaa" is set as the current value.

https://gfycat.com/SoggyBigCrow

here is the full code:

 public class TestingBug : EditorWindow {

 string abc = "0";
 int num = 0;

 [MenuItem("aaa/bbb")] 
 static void GettingWindow()
 {
     GetWindow<TestingBug>();
 }

 private void OnGUI()
 {
     abc = EditorGUILayout.TextField(abc, GUILayout.Width(200));

     num++;
     abc = num.ToString();

 }

 }