GC collects variable that is still in use

I have a small class to output debug output in the GUI:

 using UnityEngine;
 using System.Collections;
 
 public class GUIConsole : MonoBehaviour 
 {
     public int numLines = 10;
     private string message;
     private Queue queue;
 
     // Use this for initialization
     void Start () 
     {
         Application.RegisterLogCallback(OnLog);
         queue = new Queue();
     }
     
     // Update is called once per frame
     void Update () 
     {
         
     }
     
 
     void OnGUI ()
     {        
         foreach ( string line in queue )
             GUILayout.Label (line);
         
         while ( queue.Count > numLines ) queue.Dequeue();
 
     }
     
     void OnLog (string message, string callStack, LogType type)
     {
         //this.message += message + "\n";
         queue.Enqueue(message);
     }
 }

When I pause execution in Unity and resume it again, I get a null reference exception. Further investigation showed that my queue variable was suddenly null. Only after I added this line in start, it stopped:

 System.GC.KeepAlive(queue);

Any idea why the GC collects my object?

[EDIT] So, some further investigation showed: It isn't just pausing & resuming. Pausing is enough. I don't need to resume. But it only happens when I pause it, modify code and tab back to unity which auto-recompiles. This auto-recompile seems to cause the problem. It then shows the error

NullReferenceException: Object reference not set to an instance of an object GUIConsole.OnGUI () at Assets/Scripts/GUIConsole.cs:26)

a few times and then of course spams it if I resume.

[EDIT] Adding the suggestions in the answer:

 using UnityEngine;
 using System;
 using System.Collections;
 
 [Serializable]
 public class GUIConsole : MonoBehaviour 
 {
     public int numLines = 10;
 
     [SerializeField]
     public Queue queue;
 
     // Use this for initialization
     void Start () 
     {
         Application.RegisterLogCallback(OnLog);
         queue = new Queue();
     }
 
     void OnGUI ()
     {        
         foreach ( string line in queue )
             GUILayout.Label (line);
         
         while ( queue.Count > numLines ) queue.Dequeue();
 
     }
     
     void OnLog (string message, string callStack, LogType type)
     {
         queue.Enqueue(message);
     }
 }

Hower this didn't help with the problem. It is still happening