Attempt to call function 'get_var' in base 'null instance' on a null instance. - Error | Godot Café | Page 1

This is my code:

extends Node

const SAVE_FILE:String="save_file.save"
var g_data:Dictionary={}
var current_save:int=0
func _ready() -> void:
    load_data()
func save_data():
    var file:FileAccess=FileAccess.open(SAVE_FILE, FileAccess.WRITE)
    file.store_var(g_data)
    file.close()
    
func load_data():
    if not FileAccess.file_exists(SAVE_FILE):
        g_data = {
            "num_of_saves": 0
        }
    var file=FileAccess.open(SAVE_FILE, FileAccess.READ)
    g_data = file.get_var()
    file.close()

This is the error I am getting from this code:

Attempt to call function 'get_var' in base 'null instance' on a null instance.

The line that is causing this error is g_data = file.get_var(), second to last line in my code

#Attempt to call function 'get_var' in base 'null instance' on a null instance. - Error