Unassigned Reference Exception although it is assigned

Hello, there is an "Unassigned Reference Exception" in my game and it drives me mad. Apparently there are already many questions asked about the exact same problem but no answer was helpful for me. Now I will try my luck and ask the question myself:

I try to instantiate a prefab and set the canvas as its parent. This is an excerpt of my code:

 using UnityEngine;
 
 public class Inventory : MonoBehaviour
 {
     [SerializeField] Canvas canvas;
     [SerializeField] GameObject somePrefab;
 
     void Start()
     {
         canvas = GameObject.Find("Canvas").GetComponent<Canvas>();
         print(canvas.gameObject.name);
     }
 
     void Update()
     {
         print(canvas.gameObject.name);
     }
 
     public void MoveOnCanvas()
     {
         GameObject hoverObject = (GameObject)Instantiate(somePrefab);
         hoverObject.transform.SetParent(canvas.gameObject.transform, true);
     }
 }

It results in this output:

The Start() callback works fine, the canvas gets printed. Also Update() works as it should. The MoveOnCanvas() method gets called when I click on a button. When MoveOnCanvas() gets called it results in the error. The variable "canvas" is assigned, otherwise the print statements wouldn't work! It even highlights the referenced object in the Hierarchy if I click on the field in the Inspector:

I can not find a solution to this problem. I can't even find the cause of it. Many questions regarding this problem mentioned there might be a second instance of the Component (in my case "Inventory") or it might be attached to the wrong GameObject. This is not the case. I implemented a static integer variable and incremented it on Start(): Start only gets called once. As long as I get it correctly, this means there only is 1 instance of my Inventory.

If I change MoveOnCanvas() to this, it works fine:

 public void MoveOnCanvas()
     {
         GameObject hoverObject = (GameObject)Instantiate(somePrefab);
         hoverObject.transform.SetParent(GameObject.Find("Canvas").transform, true);
     }

How is this possible? Why can a member variable of a class can be assigned to some functions (Start() and Update() but contain null to others (MoveOnCanvas())? Maybe someone of you has a hint. I would appreciate it.

I could upload the whole class code to a Gist if you think it helps.