Print preview: crash when pressing a resize button

Steps to reproduce:

1. Using the WebEngine PrintMe example (with this fix already applied), click on the Print Now button to bring up the print preview window.
2. Click on the landscape button
3. Immediately after (before the preview has updated), click on one of the buttons that change the zoom factor, e.g. zoom in/out; fit width.

The result is an immediate crash in QPicture::play(), caused by an attempt to access the private class pointer which seems to already be deleted. Attached is the stack trace for the crash.

The issue is 100% reproducible, provided you click fast enough. If you instead wait for the preview to be redrawn in landscape, and then click the "fit window" button, the application will freeze instead.