title: Pyautogui TypeError: 'NoneType' object is not iterable
body: <p>I'm trying to use <code>locateCenterOnScreen()</code> function of PyAutoGUI, however it raises :</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\Users\windows\Desktop\asd.py", line 3, in &lt;module&gt;
      buttonx, buttony = pyautogui.locateCenterOnScreen('who.jpg')
  TypeError: 'NoneType' object is not iterable
  </code></pre>
  
  <p>My code is:</p>
  
  <pre><code>import pyautogui
  
  buttonx, buttony = pyautogui.locateCenterOnScreen('who.jpg')
  pyautogui.doubleClick(buttonx,buttony)
  </code></pre>
  
  <p>How can I fix this problem?</p>