Unity throws UnassignedReferenceException on a variable that is definitely assigned. Code works just fine too.

My error tells me that my camera variable is unassigned. It is a public variable that I assign from the inspector. I get the following error:

UnassignedReferenceException: The variable normalCam of motion has not been assigned.
You probably need to assign the normalCam variable of the motion script in the inspector.
Com.neon.fps.motion.FixedUpdate () (at Assets/scripts/motion.cs:45)

This confuses me because I cave used the inspector to assign the camera… and the code runs just fine with the expected result. Should I simply ignore the error?

Here is my code in case that is helpful:
will comment //ERROR HERE where the error is
please IGNORE the rest of my comments. Those are notes to self that I have figured out.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

namespace Com.neon.fps

    public class motion : MonoBehaviour
        public float speed;  //when something is public you can change it from the inspector in unity
        public float sprintMod;
        public Camera normalCam;

        private Rigidbody rig;
        private float baseFOV;
        private float sprintFOVMod = 1.25f;

        // Start is called before the first frame update
        void Start()
            //Camera.main.enabled = false;
            rig = GetComponent<Rigidbody>();
            baseFOV = normalCam.fieldOfView;

        // Update is called once per frame
        void FixedUpdate() //what does fixed update do compared to just update?
            float t_hmove = Input.GetAxisRaw("Horizontal");
            float t_vmove = Input.GetAxisRaw("Vertical");
            bool sprint = Input.GetKey(KeyCode.LeftShift); //could also add || right shift
            bool isSprinting = sprint && t_vmove > 0;

            Vector3 t_direction = new Vector3(t_hmove, 0, t_vmove);

            float t_adjustedSpeed = speed;
            if (isSprinting) t_adjustedSpeed *= sprintMod;
            rig.velocity = transform.TransformDirection(t_direction) * t_adjustedSpeed * Time.deltaTime; //apparently time.deltatime needs to be here or it defeats the purpose of using fixed update... why?

            if(isSprinting) { normalCam.fieldOfView = Mathf.Lerp(normalCam.fieldOfView, baseFOV * sprintFOVMod, Time.deltaTime * 8f); }
            else { normalCam.fieldOfView = Mathf.Lerp(normalCam.fieldOfView, baseFOV, Time.deltaTime * 8f); } //ERROR HERE

Error strangely disappeared later that night. I think it was just a Unity glitch.