Program stucks when sending serial data - python-3.x

Question

I am developing a small application, in roder to interface with my arduino.
However, in order for the arduino to start sending serial data, i have to trigger it with a specific string.
When i press the button that will send the string and will initiate the serial read, the program stucks.
No error is shown on the terminal.
Here is the relevant code:
#setting the serial object
def on_select(event=None):
global ser
COMPort = cb.get()
ser = serial.Serial(port = COMPort, baudrate=9600)
# read serial data
def readSerial():
ser_bytes = ser.readline()
ser_bytes = ser_bytes.decode("utf-8")
text.insert("end", ser_bytes)
after_id=root.after(100,readSerial)
# this function is triggered, when button 'Measure all Sensors' is pressed, on frame 2
def measure_all():
global stop
stop_ = False
ser.write(str.encode('rf')) #Send string 'rf to arduino', which means Measure all Sensors
readSerial() #Start Reading data
The first two functions work, because i gave been able to read serial data, when the arduino just spits data, without requiring activation by sending it text.
It's the third function that makes the program stuck somehow.
EDIT: This is more code - the bigger picture.
import tkinter as tk
import tkinter.ttk as ttk
import serial.tools.list_ports #for a list of all the COM ports
from tkinter import scrolledtext
#to be used on our canvas
HEIGHT = 800
WIDTH = 800
#hardcoded baud rate
baudRate = 9600
# this is the global variable that will hold the serial object value
ser = None #initial value. will change at 'on_select()'
# --- functions ---
#the following two functtions are for the seria port selection, on frame 1
#this function populates the combobox on frame1, with all the serial ports of the system
def serial_ports():
return serial.tools.list_ports.comports()
#when the user selects one serial port from the combobox, this function will execute
def on_select(event=None):
global ser
COMPort = cb.get()
string_separator = "-"
COMPort = COMPort.split(string_separator, 1)[0] #remove everything after '-' character
COMPort = COMPort[:-1] #remove last character of the string (which is a space)
ser = serial.Serial(port = COMPort, baudrate=9600)
def readSerial():
ser_bytes = ser.readline()
ser_bytes = ser_bytes.decode("utf-8")
text.insert("end", ser_bytes)
if vsb.get()[1]==1.0: #if the scrollbar is down to the bottom, then autoscroll
text.see("end")
after_id=root.after(100,readSerial)
# this function is triggered, when button 'Measure all Sensors' is pressed, on frame 2
def measure_all():
global stop_
stop_ = False
button_stop['state']='normal' #make the 'Stop Measurement' button accessible
ser.write("rf".encode()) #Send string 'rf to arduino', which means Measure all Sensors
readSerial() #Start Reading data
# --- main ---
root = tk.Tk() #here we create our tkinter window
root.title("Sensor Interface")
#we use canvas as a placeholder, to get our initial screen size (we have defined HEIGHT and WIDTH)
canvas = tk.Canvas(root, height=HEIGHT, width=WIDTH)
canvas.pack()
# --- frame 1 ---
frame1 = tk.Frame(root)
frame1.place(relx=0, rely=0.05, relheight=0.03, relwidth=1, anchor='nw') #we use relheight and relwidth to fill whatever the parent is - in this case- root
label0 = tk.Label(frame1, text="Select the COM port that the device is plugged in: ")
label0.config(font=("TkDefaultFont", 8))
label0.place(relx = 0.1, rely=0.3, relwidth=0.3, relheight=0.5)
cb = ttk.Combobox(frame1, values=serial_ports())
cb.place(relx=0.5, rely=0.5, anchor='center')
cb.bind('<<ComboboxSelected>>', on_select)
# --- frame 1 ---
# --- frame 2 ---
frame2 = tk.Frame(root, bd=5) #REMOVED THIS bg='#80c1ff' (i used it to see the borders of the frame)
frame2.place(relx=0, rely=0.1, relheight=0.07, relwidth=1, anchor='nw')
#Button for 'Measure All Sensors'
#it will be enabled initially
button_all = tk.Button(frame2, text="Measure all Sensors", bg='#80c1ff', fg='red', state='normal', command=measure_all) #bg='gray'
button_all.place(relx=0.2, rely=0.5, anchor='center')
#frame 6 will be the area with the texct field
# --- frame 6 ---
frame6 = tk.Frame(root, bg='#80c1ff') #remove color later
frame6.place(relx=0.0, rely=0.4, relheight=1, relwidth=1, anchor='nw')
text_frame=tk.Frame(frame6)
text_frame.place(relx=0, rely=0, relheight=0.6, relwidth=1, anchor='nw')
text=tk.Text(text_frame)
text.place(relx=0, rely=0, relheight=1, relwidth=1, anchor='nw')
vsb=tk.Scrollbar(text_frame)
vsb.pack(side='right',fill='y')
text.config(yscrollcommand=vsb.set)
vsb.config(command=text.yview)
# --- frame 6 ---
stop_=True # Stop Flag. True when no measuring is happening
root.mainloop() #here we run our app
# --- main ---