Pythonで、referenced before assignmentというエラーが出る

前提

PythonでTkinterライブラリを使用してゲームを制作していたところ
referenced before assignment
というエラーが表示されてしまいました。

実現したいこと

エラー解消

発生している問題・エラーメッセージ

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Users\patpw\AppData\Local\Programs\Python\Python310\lib\tkinter\__init__.py", line 1921, in __call__
    return self.func(*args)
  File "c:\Python\untigame\sikou.py", line 101, in start_game
    decode_line(None)
  File "c:\Python\untigame\sikou.py", line 14, in decode_line
    if current_line >=len(scenario):
UnboundLocalError: local variable 'current_line' referenced before assignment

該当のソースコード

Python
1import tkinter
2
3#スタートメニュー
4def start_menu():
5    st_btn = tkinter.Button(text = "はじめる",width = 20,back = "blue")
6    st_btn.place(x = 300,y = 100)
7    #スタートゲーム呼び出し
8    st_btn["command"] = start_game
9
10#シナリオ読み込み
11def decode_line(event):
12    global bgimg, lcharimg, ccharimg, rcharimg
13    #最終行到達で終了
14    if current_line >=len(scenario):
15        return;
16    #行を読み込んで次へ移動
17    line = scenario[current_line]
18    current_line=current_line + 1
19    line=line.replace("\\n","\n").strip()
20    params=line.split(" ")
21
22    if line[0] != "#":
23        message["text"]=line
24        return
25    elif params[0] == "#back":
26        canvas.delete("all")
27        bgimg = tkinter.PhotoImage(file = params[1])
28        canvas.create_image(450,230,image=bgimg)
29    elif params[0] == "#putChar":
30        if params[2] == "L":
31            canvas.delete("left")
32            lcharimg=tkinter.PhotoImage(file=params[1])
33            canvas.create_image(200,160,image=lcharimg,tag="left")
34        elif params[2] == "R":
35            canvas.delete("right")
36            rcharimg=tkinter.PhotoImage(file=params[1])
37            canvas.create_image(700,160,image=lcharimg,tag="right")
38        else: 
39            canvas.delete("center")
40            ccharimg=tkinter.PhotoImage(file=params[1])
41            canvas.create_image(450,160,image=lcharimg,tag="center")
42
43    elif params[0] == "#delChar":
44        if params[2] == "L":
45            canvas.delete("left")
46        elif params[2] == "R":
47            canvas.delete("right")
48        else:
49            canvas.delete("center")
50    elif params[0] == "#branch":
51        message.unbind("<Button-1>")
52        btn = tkinter.Button(text=params[2],width=20)
53        branch.append(btn)
54        btn["command"] = lambda : jump_to_line(int(params[1]) - 1)
55        btn.place(x=300,y=60+int(params[1])*60)
56        jumplabel.append(params[3])
57        if params[4] == "n":
58            return
59    elif params[0] == "#jump":
60        label = params[1].strip()
61
62        for l in range(len(scenario)):
63            if scenario[l].strip() == "## " + label:
64                current_line = l
65                decode_line(None)
66                return
67    elif params[0].strip() == "#end":
68        message["text"] = "owari"
69        message.unbind("<Button-1>")
70        current_line = 99999999
71        
72    decode_line(None)
73
74def jump_to_line(branchID):
75    for btn in branch:
76        btn.place_forget()
77        btn.destroy()
78    branch.clear()
79    label = jumplabel[branchID]
80    jumplabel.clear()
81    message.bind("<Button-1>",decode_line)
82
83    for l in range(len(scenario)):
84        if scenario[l].strip() == "## " + label:
85            current_line = l
86            decode_line(None)
87            return
88
89root = tkinter.Tk()
90root.title("思考述べる")
91root.minsize(900,460)
92root.option_add("*font",["メイリオ",14])
93
94canvas = tkinter.Canvas(width=900,height=460)
95canvas.place(x=0,y=0)
96
97
98
99#ゲームスタート
100def start_game():
101    decode_line(None)
102    #ここでcurrent_lineを宣言
103    global current_line 
104    current_line = 0
105    global scenario 
106    scenario = []
107    file=open("senario.txt","r",encoding="utf-8")
108    while True:
109        line=file.readline()
110        scenario.append(line)
111        if not line:
112            file.close()
113            break
114
115    global message 
116    message = tkinter.Label(width=70,height=5,wraplength=840,
117        bg="white",justify="left",anchor="nw")
118
119    message.place(x=28,y=284)
120    message["text"]="クリックしてスタート"
121    message.bind("<Button-1>",decode_line)
122
123start_menu()
124
125bgimg= None
126lcharimg = None
127ccharimg = None
128rcharimg = None
129branch = []
130jumplabel = []
131
132root.mainloop()