I cannot save my game data on Godot Engine

Question

I'm trying to save my game on Godot Engine by pressing a "save" button but I get this message error when i do:

Attempt to call function 'get_nodes_in_group' in base 'null instance' on a null instance

this is my project code:

#File.gd, contains the methods to write and read over the file
extends Node

var path_name := "res://resource/debug/save"
var file := "sav.tres"


class_name WriterReader

func write():
    var writer := Save.new()
    var directory := Directory.new()

    writer.version = ProjectSettings.get_setting("application/config/Version")
    for node in get_tree().get_nodes_in_group("persistance"): #it's  the line that thows the error
        node.save(writer)
    print(writer.version)

    if not directory.dir_exists(path_name):
        directory.make_dir_recursive(path_name)

    if ResourceSaver.save(path_name.plus_file(file), writer) != OK:
        print("ERROR")


#the save button gdscript file
extends Button


func _ready():
    var save = Save.new()
    self.connect("pressed", self, "save_game", [save])


func save_game(save):
    var s = WriterReader.new()
    s.write()



#it's the part of code where of the player character is saving 
extends KinematicBody2D
...

func save(write: Resource):
    write.dat = {"position": position, "health": health}

#sav.gd, singleton used to save the player data

extends Resource

class_name Save

var dat : Dictionary = {}
var version := ""

I'm a beginner on Godot so I cannot figure out why this happens.

could somebody explain me, please?

Thanks

Answer 1

Your WriterReader is calling get_tree while it is not part of the SceneTree. You could do something like this:

func save_game(save):
    var s = WriterReader.new()
    add_child(s)
    s.write()

Since it looks like your WriterReader isn't designed to be part of the scene tree, it might make more sense to pass the objects into write from outside:

# in WriterReader.gd:
static func write(nodes: Array):
    # ...
    for node in nodes:
        node.save(writer)
    # ...

# in Button.gd:
func save_game(save):
    # this should work, as your Button is part of the SceneTree
    WriterReader.write(get_tree().get_nodes_in_group("persistance"))