Unplugging the external screen attached to a closed MacBookPro, causes all running Qt applications to crash

Description

When you unplug the external screen attached to a closed MacBookPro, all running Qt applications are crashing.

Reproducibility

• This issue is reproducible with Qt 5.13.1 on macOS 10.15.1

Steps to reproduce

1. Attach an external display to a MacBook Pro. I used a Thunderbolt Dock and Display Port to connect the external display.
2. Close the MacBook Pro
3. Launch any Qt application on the external screen. For example Qt Creator or Linguist.app.
4. Unplug the Thunderbold cable of the external display
5. Open the lid after having unplugged the cable and with the cable still unplugged

Result: All running Qt applications (for example Qt Creator or Linguist.app) have crashed

More info

 The crash occurs in windowWillUseStandardFrame:defaultFrame: in qnswindowdelegate.mm.

As you can see in this code, there is no check for NULL pointer for platformWindow->screen() which is I think causing the crash:

    const QRectF screenGeometry = platformWindow->screen()->geometry();

Qt Support:

Tested the steps to reproduce with similar equipment. Could not reproduce the issue. We suspect that some setting or other unknown is causing this.