Try to solve "(Python) tkinter figures (with colorbar) overlap when using slider "

For my old problem (Python) tkinter figures (with colorbar) overlap when using slider (I add bounty to this problem):

I tried to solve it by considering the following good suggestion:

tkinter figure (with colorbar) shrinks every time it's displayed

However, the new problem popped out. I rewrote the update() function as the following

def update(val):
    num_on_slider.append(slider_de.val)
    for ii in range(0,num):
        if num_on_slider[-1] == ii:
                global cbar    # <-------- new
                if cbar:       # <-------- new
                    cbar.remove()       # <-------- new
                    con = ax.contourf(x,y,dfs[ii], levels = levels, cmap=cm.jet, alpha = 0.5, antialiased = True)
                    cbar = fig.colorbar(con,ax = ax)
                    ax.axis([1, 12, 1, 6])

No overlap; however, the graph will not change as I move the slider.
It looks like the condition if cbar: block the following part of codes being executed.

But in my original problem, I assign cbar outside the function

    cbar = fig.colorbar(con,ax = ax)
    ax.axis([1, 12, 1, 6])
    # ================================================Slider==========================================
    global slider_de
    slider_bar = fig.add_axes([0.12, 0.1, 0.78, 0.03])     
    slider_de = Slider(slider_bar, 's_bar', 0, num-1, valinit=1,valfmt='%0.0f',  valstep=1)
    num_on_slider = []

    def update(val):
        num_on_slider.append(slider_de.val)
        for ii in range(0,num):
            if num_on_slider[-1] == ii:
                    global cbar
                    if cbar:
                        cbar.remove()
                        con = ax.contourf(x,y,dfs[ii], levels = levels, cmap=cm.jet, alpha = 0.5, antialiased = True)
                        cbar = fig.colorbar(con,ax = ax)
                        ax.axis([1, 12, 1, 6])
                
    slider_de.on_changed(update)          

So the value of cbar should not be None.

How to fix the problem?

Thanks!