title: gtk-builder-error-quark: invalid object type 'WebKitWebView'
body: <p>I am getting the following error when trying to get a <code>WebKitWebView</code> object via <code>Gtk.builder()</code>:</p>
  
  <pre><code>$ python3 test.py
  Traceback (most recent call last):
    File "test.py", line 11, in &lt;module&gt;
      builder.add_from_file("ui.glade")
  gi.repository.GLib.Error: gtk-builder-error-quark: ui.glade:31:1 Invalid object type 'WebKitWebView' (6)
  </code></pre>
  
  <p>Here's my Python code</p>
  
  <pre><code>#!/usr/bin/env python
  
  import gi
  gi.require_version('WebKit2', '4.0')
  gi.require_version('Gtk', '3.0')
  
  from gi.repository import Gtk
  from gi.repository import WebKit2 as Webkit
  
  builder = Gtk.Builder()
  builder.add_from_file("ui.glade")
  
  window = builder.get_object("window")
  window.set_title("Test")
  
  webview = builder.get_object("webview")
  
  if __name__ == "__main__": 
    window.show_all()
    Gtk.main()
  </code></pre>
  
  <p>And here's my <code>ui.glade</code> file:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;!-- Generated with glade 3.22.1 --&gt;
  &lt;interface&gt;
    &lt;requires lib="gtk+" version="3.20"/&gt;
    &lt;requires lib="webkit2gtk" version="2.12"/&gt;
    &lt;object class="GtkWindow" id="window"&gt;
      &lt;property name="width_request"&gt;800&lt;/property&gt;
      &lt;property name="height_request"&gt;600&lt;/property&gt;
      &lt;property name="can_focus"&gt;False&lt;/property&gt;
      &lt;child&gt;
        &lt;placeholder/&gt;
      &lt;/child&gt;
      &lt;child&gt;
        &lt;object class="GtkBox"&gt;
          &lt;property name="visible"&gt;True&lt;/property&gt;
          &lt;property name="can_focus"&gt;False&lt;/property&gt;
          &lt;property name="orientation"&gt;vertical&lt;/property&gt;
          &lt;child&gt;
            &lt;object class="GtkEntry"&gt;
              &lt;property name="visible"&gt;True&lt;/property&gt;
              &lt;property name="can_focus"&gt;True&lt;/property&gt;
            &lt;/object&gt;
            &lt;packing&gt;
              &lt;property name="expand"&gt;False&lt;/property&gt;
              &lt;property name="fill"&gt;True&lt;/property&gt;
              &lt;property name="position"&gt;0&lt;/property&gt;
            &lt;/packing&gt;
          &lt;/child&gt;
          &lt;child&gt;
            &lt;object class="WebKitWebView" id="webview"&gt;
              &lt;property name="visible"&gt;True&lt;/property&gt;
              &lt;property name="can_focus"&gt;True&lt;/property&gt;
              &lt;child&gt;
                &lt;placeholder/&gt;
              &lt;/child&gt;
            &lt;/object&gt;
            &lt;packing&gt;
              &lt;property name="expand"&gt;False&lt;/property&gt;
              &lt;property name="fill"&gt;True&lt;/property&gt;
              &lt;property name="position"&gt;1&lt;/property&gt;
            &lt;/packing&gt;
          &lt;/child&gt;
        &lt;/object&gt;
      &lt;/child&gt;
    &lt;/object&gt;
  &lt;/interface&gt;
  </code></pre>
  
  <p>There's a similar question <a href="https://stackoverflow.com/questions/51010820/glade-invalid-object-type-webkitwebview-when-loading-from-file-works-on-gla">here</a>, but I honestly didn't "get the point" as the answer said.</p>