Why can't I click the button from the matplotlib example code when running in PyCharm?

Question

I want to use Button with from matplotlib.widgets import Button. But it just plots the button and I can't click it.

It is the example of button in matplotlib site but it doesn't work. My IDE is PyCharm.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.widgets import Button

freqs = np.arange(2, 20, 3)

fig, ax = plt.subplots()
plt.subplots_adjust(bottom=0.2)
t = np.arange(0.0, 1.0, 0.001)
s = np.sin(2*np.pi*freqs[0]*t)
l, = plt.plot(t, s, lw=2)


class Index:
   ind = 0

   def next(self, event):
       self.ind += 1
       i = self.ind % len(freqs)
       ydata = np.sin(2*np.pi*freqs[i]*t)
       l.set_ydata(ydata)
       plt.draw()

   def prev(self, event):
       self.ind -= 1
       i = self.ind % len(freqs)
       ydata = np.sin(2*np.pi*freqs[i]*t)
       l.set_ydata(ydata)
       plt.draw()

callback = Index()
axprev = plt.axes([0.7, 0.05, 0.1, 0.075])
axnext = plt.axes([0.81, 0.05, 0.1, 0.075])
bnext = Button(axnext, 'Next')
bnext.on_clicked(callback.next)
bprev = Button(axprev, 'Previous')
bprev.on_clicked(callback.prev)

plt.show()