Instantiating Custom Object in Unity MonoBehaviour Scripts

I'm trying to programm my own little game in Unity, I'm not new to programming but new to Unity and C#. The problem is that I wrote my own classes to handle the game map and I am trying to connect these classes to the MonoBehaviour Scripts in Unity.

I have a script called InputManager, which is supposed to handle the input via mouse and keyboard etc. and in this script I want to create an object of the class MapManager which has access to my tiles and stuff. The problem I'm getting is that I can't seem to create an instance of MapManager in InputManager, not with the common new MapManager() anyway. When I use this a NullPointer of some sort is created, I guess?

I do not get a compiling error but an error once the game is launched which is:

 NullReferenceException: Object reference not set to an instance of an object
 InputManager.Update () (at Assets/Scripts/InputManager.cs:55) 
 

Thank you for your help!

!UPDATE!:

I tried using a workaround so that it does not matter if Start() or Update() is called first, meaning that I just instantiate MapManager in the first Update() call. However, when I run the program the error is still the same. My conclusion is that somehow my constructor is not working or the Monobehaviour script somehow does not allow using the standard constructor... Any ideas?

Here is my code:

InputManager.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Tilemaps;
 
 public class InputManager : MonoBehaviour
 {
     public int mapRightBorder = 10;
     public int mapLeftBorder = -10;
     public int mapTopBorder = 8;
     public int mapBottomBorder = -4;
 
     public double camSpeed = 0.2;
     public double mouseSensitivity = 0.3;
     public double scrollSensitivity = 2;
 
     public double maxZoomIn = -3;
     public double maxZoomOut = -12;
 
     public Tilemap ground, overlays, buildings;
 
     public Tilemap selected;
 
     public AnimatedTile animatedTile;
 
     private double detectionBorder;
     private double leftRightCorrection;
 
     private MapManager mapManager;
 
     // Start is called before the first frame update
     void Start()
     {
         detectionBorder = 50;
         leftRightCorrection = 1.2;
 
         mapManager = new MapManager();
 
         mapManager.ground = ground;
         mapManager.overlays = overlays;
         mapManager.buildings = buildings;
 
         mapManager.selected = selected;
 
         mapManager.animatedTile = animatedTile;
 
         Cursor.lockState = CursorLockMode.Confined;
     }
 
     // Update is called once per frame
     void Update()
     {
         moveCam();
         checkMouse();
         mapManager.updateTilemaps();
     }
 
     private void moveCam()
     {
         double moveX = Camera.main.transform.position.x;
         double moveY = Camera.main.transform.position.y;
         double moveZ = Camera.main.transform.position.z;
         
         double xPos = Input.mousePosition.x;
         double yPos = Input.mousePosition.y;
         double zDelta = Input.GetAxis("Mouse ScrollWheel");
 
         if (Input.GetKey(KeyCode.LeftArrow))
         {
             moveX -= camSpeed * leftRightCorrection;
         }
         else if (xPos >= 0 && xPos < detectionBorder)
         {
             moveX -= camSpeed * mouseSensitivity * leftRightCorrection;
         }
         else if (Input.GetKey(KeyCode.RightArrow))
         {
             moveX += camSpeed * leftRightCorrection;
         }
         else if (xPos <= Screen.width && xPos > Screen.width - detectionBorder)
         {
             moveX += camSpeed * mouseSensitivity * leftRightCorrection;
         }
 
         if(moveX > mapRightBorder || moveX < mapLeftBorder)
         {
             moveX = Camera.main.transform.position.x;
         }
 
 
         if (Input.GetKey(KeyCode.DownArrow))
         {
             moveY -= camSpeed;
         }
         else if (yPos >= 0 && yPos < detectionBorder)
         {
             moveY -= camSpeed * mouseSensitivity;
         }
         else if (Input.GetKey(KeyCode.UpArrow))
         {
             moveY += camSpeed;
         }
         else if (yPos <= Screen.height && yPos > Screen.height - detectionBorder)
         {
             moveY += camSpeed * mouseSensitivity;
         }
 
         if(moveY > mapTopBorder || moveY < mapBottomBorder)
         {
             moveY = Camera.main.transform.position.y;
         }
 
 
         if (!(moveZ + zDelta * scrollSensitivity > maxZoomIn || moveZ + zDelta * scrollSensitivity < maxZoomOut))
         {
             moveZ += zDelta * scrollSensitivity;
         }
 
         Vector3 newPos = new Vector3((float)moveX, (float)moveY, (float) moveZ);
         Camera.main.transform.position = newPos;
     }
 
     private void checkMouse()
     {
         if (Input.GetMouseButtonDown(0))
         {
             mapManager.selectTile(Input.mousePosition);
         }
     }
 }
 

MapManager.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Tilemaps;
 
 public class MapManager
 {
     public int mapWidth = 16;
     public int mapHeight = 16;
 
     public Tilemap ground, overlays, buildings;
 
     public Tilemap selected;
 
     public AnimatedTile animatedTile;
 
     private Vector3Int currentlySelected;
 
     private OwnTilemap tilemap;
 
     public MapManager()
     {   
         tilemap = new OwnTilemap(mapWidth, mapHeight, ground, overlays, buildings);
         currentlySelected = new Vector3Int(-1, -1, 0);
     }
 
     private Vector3Int mouseToTilemap(Vector3 mousePosition)
     {
         Ray ray = Camera.main.ScreenPointToRay(mousePosition);
         // create a plane at 0,0,0 whose normal points to +Y:
         Plane hPlane = new Plane(Vector3.forward, Vector3.zero);
         // Plane.Raycast stores the distance from ray.origin to the hit point in this variable:
 
         float enter = 0.0f;
         if (hPlane.Raycast(ray, out enter))
         {
             //Get the point that is clicked
             Vector3 hitPoint = ray.GetPoint(enter);
             Vector3Int cell = ground.WorldToCell(hitPoint);
 
             return cell;
         }
 
         return new Vector3Int(-1, -1, 0);
     }
 
     private bool isInBorders(Vector3Int p)
     {
         if (p.z != 0) return false;
         if (p.x < 0 || p.x >= mapWidth) return false;
         if (p.y < 0 || p.y >= mapHeight) return false;
         return true;
     }
 
     public void updateTilemaps()
     {
         tilemap.updateTilemaps();
     }
 
     public void previewBuilding()
     {
 
     }
 
     public void selectTile(Vector3 mousePosition)
     {
         if(currentlySelected != new Vector3Int(-1, -1, 0))
         {
             selected.SetTile(currentlySelected, null);
         }
         if (isInBorders(mouseToTilemap(mousePosition)))
         {
             selected.SetTile(mouseToTilemap(mousePosition), animatedTile);
             currentlySelected = mouseToTilemap(mousePosition);
         }
     }
 }