title: What is the required value for the Window argument in RenderView
body: <p>I am busy migrating a project to null-safety and am running into an issue with the following code snippet:</p>
  <pre><code>RenderView(
   child: RenderPositionedBox(
   alignment: Alignment.center,
   child: repaintBoundary,
   ),
   configuration: ViewConfiguration(
   size: logicalSize,
   devicePixelRatio: 1.0,
   ),
   window: null,
  )
  </code></pre>
  <p>The <code>window</code> argument is not nullable and requires a <code>FlutterView</code> which is an abstract class, google is not helping me figure out what actually needs to be used there.</p>
  <p>Tx</p>