Null Reference Exception only in second method

Questions & Answers
1

I’ve had several Null Reference Exception errors during the development of this project, and I felt like I had a handle on why they were happening, but this one has me stumped. I have a prefab (called specialFind) that gets instantiated several times throughout the scene on load. It has the following script on it (note that I took out several variables to save on space here, but if my comments aren’t clear, I can provide the full code):

using UnityEngine;
using UnityEngine.UI;
using System.Collections;

public class SFdata : MonoBehaviour {
    //public variables that receive data at instantiation
    public int ID;  //there are several others

    CamMouseLook mouselook;
    public GameObject viewer;
    public Canvas viewerCanvas;

    //Record Viewer Fields declared
    private Text viewerID;  //again, several others exist

    void Start()
    {
        viewer = GameObject.Find("recordViewer");
        viewerCanvas = viewer.GetComponent<Canvas>();
        viewerCanvas.enabled = false;
        Debug.Log("viewer is disabled");
        mouselook = GameObject.Find("Main Camera").GetComponent<CamMouseLook>();

        //assign correct components to record viewer fields
        viewerID = viewer.transform.Find("Panel/Header").GetComponent<Text>();
        //again, there are others
    }
    void OnMouseDown()
    {
        //when object is clicked on, open the dataViewer window and show the relevant data
        mouselook.enabled = false;
        Cursor.lockState = CursorLockMode.None;
        Cursor.visible = true;
        viewerCanvas.enabled = true;
        Debug.Log("Viewer is enabled");

        //set values for viewer
        viewerID.text = "Special Find: " + ID.ToString();
        //several others        
    }
    public void closeRecordViewer()
    {
        SFdata SFdata = this;
        Cursor.lockState = CursorLockMode.Locked;
        Cursor.visible = false;
        //next line throws Null Reference Exception
        mouselook.enabled = true;
        viewerCanvas.enabled = false;
    }
}

So right now, things work as intended for the first two steps. If you click on an instance of specialFind it will open the data viewer and display the proper data. The problem happens when I click the ‘X’ button I made (grandchild of viewerCanvas), which has an OnClick event that calls closeRecordViewer(). For some reason, Start() and OnMouseDown() can use the variables like mouselook and viewerCanvas, but closeRecordViewer() gets a NRE when trying to access them. I thought that, being inside the same class, all methods would have access to these variables, but maybe I’m wrong? Can anyone tell me why this is happening?

Also, if you’ve got any criticism about the structure of the code, I’d very much appreciate that as well. But I mostly just want to get this thing working.

2

Sounds like you have two components, of the same class (SFdata), on two or more objects in the Scene.
Since you declared the variables without the “static” modifier, each instance of this class will store it’s own copy of these variables. This makes sense when your class is say… a text field, and need to store a unique string.
However, if you want ALL instances of your class to be able to access the SAME variable, you will need to define the variable as static.

e.g.

public class SFdata : MonoBehaviour {
     //public variables that receive data at instantiation
     public int ID;  //looks like this one SHOULD be stored in each instance.
 
     static CamMouseLook mouselook;  //I suspect this is one you wan to make static, and share this one variable among all instances of SFData
     public GameObject viewer;  // not sure if you'd want this to be the same of all instances....
     static public Canvas viewerCanvas; // I'd guess this one should be static too.
     ....
     }

If you choose to NOT make them static, you will need to ensure that every instance has it’s variables setup properly when created or enabled.

e.g.

OnEnable()
{
   if(!mouselook || !viewerCanvas)
        Debug.Log("Warning, not all variables setup on object: " + gameObject.name);
}