Address details window cannot be closed

When I use Electrum and open the 'Details' window for an address, after click the QR code icon, the 'Details' window will not be able to be closed.

How to reproduce this issue

• Run Electurm
• Click 'Addresses' tab shown in the red box of the screenshot
  
• Select an address and right click to bring up the following menu
  
• Click the 'Details' item in the menu shown in the above screenshot. It will bring up the Details window
• Clock the QR icon in the Details window. The icon is highlighted in a red box. After the icon clicked nothing happened.
  
• Click 'Close' button in the above window. The window cannot be closed.

If we do not click the QR icon before we click the 'Close' button, then the window can be closed.

If we click the QR icon, then the 'Details' window will be be able to be closed and we cannot go back to the main window.

The possible reason is that QR icon tried to open a new window, but failed. So the 'Details' window are not on the top, so it cannot be closed.

Environment

• Computer: MacBook
• OS: macOS Sierra version 10.12.6
• Eletrum version: 3.0.2