title: RenderBox size returns 0 even after WidgetsBinding addPostFrameCallback
body: <p>So I'm developing an application where the user picks an image (either from camera or gallery) and I need to get the size of the widget where this image is loaded to draw some lines on top of it.</p>
  <p>The problem is that I'm trying to get RenderBox.size of this widget on WidgetsBinding.instance.addPostFrameCallback and it has 0 width and height.</p>
  <p>Some code:</p>
  <p><code>WidgetsBinding.instance.addPostFrameCallback((_) {</code><br>
  <code>  final RenderBox renderBox = _keyPic.currentContext.findRenderObject();</code><br>
  <code>  final size = renderBox.size;</code><br>
  <code>});</code></p>
  <p>Image is loaded using the code below:</p>
  <p><code>Image.memory(_file.readAsBytesSync(), key: _keyPic)</code></p>
  <p>I'm really not sure what's going on, if someone can shed a light on this issue would be truly appreciated!</p>