title: In TKinter, why does tkMessageBox bar me from modifying Entry objects?
body: <p>I'm using TKinter and I have a really weird situation. </p>
  
  <p>I've basically created a small application that has Listbox, two Buttons, and an Entry. The two Buttons add and remove items from the Listbox, respectively, and the Entry specifies the name of the item added. This is all encapsulated in a Frame.</p>
  
  <p>To the right of this Frame is a near identical one, except the Listbox of this Frame contains items that are parented to the ones on the left. Basically, 'lists' or 'categories' are created in the first Listbox, and then items are added to those lists in the second Listbox.</p>
  
  <p>TL;DR: <a href="http://imgur.com/a/5BKm6" rel="nofollow">Here are three screenshots of the application in use, to make things easier to visualize.</a></p>
  
  <p>In my application, there can be no two categories of the same name. If there are two categories of the same name, tkMessagebox.showerror(...) is called. This works correctly and my program still functions correctly, <em>except I cannot highlight or modify either of the Entry fields anymore.</em></p>
  
  <p>I at first thought they were for some reason being set to DISABLED or readonly, but I know this cannot be because the buttons of my program still work, and when I press 'Add Category', the contents of its Entry field are copied to the Categories list and then removed from the Entry field. Entry fields are not modifiable, even via code (.delete(...) and .insert(...) calls don't work) when set to DISABLED or readonly, yet my application is clearly able to use these calls.</p>
  
  <p>Furthermore, when I mouse over the Entry fields, my cursor switches to the 'reading cursor' (like when you mouse over this text). When I attempt to click, click and drag, or type, nothing happens. It's as if the Entry fields have permanently lost focus.</p>