title: Runtime error when category is added to the UIWebView
body: <h1>Description</h1>
  
  <p>I just add a category <code>UIWebView+CategoryTest</code> to <code>UIWebView</code>, which swizzling it's <code>layoutSubviews</code> with a custom method <code>hlz_layoutSubviews</code>, but the <code>UIWebView</code> never used in my project. When I build and run the application, a runtime error shows up when excuting <code>[self hlz_layoutSubviews];</code></p>
  
  <blockquote>
    <p>CategoryTest[29881:2122091] -[UIWindow hlz_layoutSubviews]: unrecognized selector sent to instance 0x7f8e73d311e0</p>
  </blockquote>
  
  <p>Am I missing something?</p>
  
  <h1>Steps to reproduce the issue</h1>
  
  <ol>
  <li>Create an empty project using <code>Single View Application</code> template in Xcode.</li>
  <li>Add a new category <code>UIWebView+CategoryTest</code> to <code>UIWebView</code>.</li>
  <li>Replace the code in <code>UIWebView+CategoryTest.m</code> with followings:</li>
  </ol>
  
  <p>```</p>
  
  <pre><code>@implementation UIWebView (CategoryTest)
  
  + (void)load {
      // Swizzle the `layoutSubviews` method.
      static dispatch_once_t onceToken;
      dispatch_once(&amp;onceToken, ^{
          Method originalMethod = class_getInstanceMethod(self, @selector(layoutSubviews));
          Method swizzledMethod = class_getInstanceMethod(self, @selector(hlz_layoutSubviews));
  
          method_exchangeImplementations(originalMethod, swizzledMethod);
      });
  }
  
  - (void)hlz_layoutSubviews {
      [self hlz_layoutSubviews];
  }
  
  @end
  </code></pre>
  
  <p>```</p>
  
  <ol start="4">
  <li>Add <code>Exception Breakpoint</code> for <code>All Exceptions</code> in the <code>Breakpoint navigator</code>.</li>
  <li>Build and run the demo application.</li>
  </ol>