Problems with matplotlib and tkinter: Exception in Tkinter callback

From the docs of plt.show() you can see that what this function does is:

display all figures and block

Because Tkinter relies on the mainloop to run, this doesn't work well with blocking functions such as plt.show().

Again from the docs:

A single experimental keyword argument, block, may be set to True or False to override the blocking behavior described above.

Which means you can call plt.show(block=False) to make it not block the Tkinter mainloop.

However, there is a way neater option to integrate matplotlib in Tkinter, an example can be found here. In the most simple implementation, this would translate to your code something like this:

from tkinter import *
import numpy as np

from matplotlib.figure import Figure
from matplotlib.backends.backend_tkagg import (FigureCanvasTkAgg, NavigationToolbar2TkAgg)

class MyGUI:
    def __init__(self):
        win = Tk()
        win.geometry('750x640')
        win.resizable(False,False)
        win.update()
        w = win.winfo_width()
        h = win.winfo_height()

        btn1 = Button(win,text='Enable button 2',command=self.btn1Listener)
        btn1.place(x=20, y=20)
        btn1.update()

        self.btn2 = Button(win,text='Plot 1',command=self.btn2Listener)
        self.btn2.place(x=btn1.winfo_x()+btn1.winfo_width()+10, y=20, width=150)
        self.btn2.configure(state='disabled')
        self.btn2.update()

        self.btn3 = Button(win,text='Plot 2',command=self.btn3Listener)
        self.btn3.place(x=self.btn2.winfo_x()+self.btn2.winfo_width()+10, y=20, width=150)
        self.btn3.configure(state='disabled')
        self.btn3.update()


        self.fig = Figure(figsize=(5, 4), dpi=100)
        self.ax = self.fig.add_subplot(111)

        self.canvas = FigureCanvasTkAgg(self.fig, master=win)  # A tk.DrawingArea.
        self.canvas.draw()
        self.canvas.get_tk_widget().place(x=20, y=80)

        win.mainloop()

    #------------------------------------------------------- 
    def btn1Listener(self):
        self.btn2.configure(state='normal')

    def btn2Listener(self):
        self.s, self.v = self.B1()
        self.btn3.configure(state='normal')

    def btn3Listener(self):
        self.B2(self.s, self.v)

    #-------------------------------------------------------------
    def B1(self):                  
        x=np.arange(0, 5, 0.1)
        y1=np.sin(x)
        self.ax.plot(x, y1)
        self.canvas.draw()
        return 0,5 

    def B2(self, s, v):
        x=np.arange(s, v, 0.1)
        y1=np.cos(x)
        # Use this if you want to remove the first plot
        # self.ax.clear()
        self.ax.plot(x, y1)
        self.canvas.draw()
        self.fig.savefig('aaa.png')


#------------------------------------------------------------------        
if __name__ =='__main__':   
    MyApp=MyGUI()

P.S. I wouldn't recommend using place because it gets very difficult to manage in the long run and doesn't like resizing windows. Try setting up your GUI using grid and/or pack instead.